Dictionary category: Definition

About this dictionary

This DICTIONARY OF LIGHT, COLOUR & VISION contains a vocabulary of closely interrelated terms that underpin all the resources you will find here at lightcolourvision.org.

  • Each term has its own page in the DICTIONARY and starts with a DEFINITION.
  • Bullet points follow that provide both context and detail.
  • Links embedded in the text throughout the site (highlighted in blue) take you directly to DICTIONARY entries.
  • Shorter SUMMARIES of terms appear on DIAGRAM PAGES under the heading SOME KEY TERMS. These entries strip definitions back to basics and can be viewed without leaving the page.
Why a dictionary of light, colour & vision
  • One of the practical objectives of this website is to make the connections between the topics of light, colour and vision accessible to students and researchers of all ages.
  • Our DICTIONARY aims to avoid a problem faced by websites such as Wikipedia where articles are often composed by contributors with narrow specialisation and their own topic-specific vocabulary.
  • The layout of the DICTIONARY also aims to avoid situations where a single unknown word or phrase makes it difficult, if not impossible, for our visitors to find the information they need (as explained below).
Terms, definitions and explanations
  • All the terms we have selected for the DICTIONARY are widely used and are applied consistently across the topics of light, colour and vision.
  • The aim is to avoid definitions and explanations with different meanings in different fields.
  • As far as possible definitions contain no more than two short sentences.
  • The explanations that follow each definition are arranged as short bullet points that avoid paragraphs of information completely.
  • Each bullet makes a stand-alone point and is intended to deal with a single piece of information that we believe is likely to be important to our readership.
  • The writing style across all terms aims to be clear, accessible and engaging.
  • The idea is to enable our visitors to find and digest information quickly and to confirm facts one at a time.
  • Because our readership and their concerns are diverse, bullet points sometimes provide different perspectives on a single term or topic.

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Absorption

When light strikes an object, some wavelengths are absorbed and their energy is converted to heat, others undergo reflection or transmission.

  • When light is absorbed by an object or medium, its energy excites electrons, which emit heat.
  • Absorption of a particular wavelength of light by a material occurs when the frequency of the wave matches the resonant frequency of electrons orbiting atomic nuclei in the material.
  • The opposite of absorption is transmission which refers to the process of electromagnetic radiation passing through a medium. When electromagnetic waves move through a material without being absorbed or reflected, we say they are transmitted. If no radiation is reflected or absorbed at all, the material achieves 100% transmission.
  • Electrons selectively absorb photons whose frequencies match their resonant frequencies.
  • Resonant frequency refers to the frequency at which electrons are most efficiently excited by absorbed light, leading to the emission of heat.
  • As electrons absorb energy from photons, they vibrate more vigorously, leading to collisions between atoms and the production of heat.
  • When light is reflected from a surface, it bounces off at the same wavelength with little or no change in energy.
Related diagrams

Each of the diagrams below can be viewed on its own page with a full explanation.

References
  • When light is absorbed by an object or medium, its energy excites electrons, causing them to vibrate more vigorously and to collide with other atoms, which in turn produces heat.
  • Materials selectively absorb photons whose frequencies match their own frequencies.
  • Absorption of light occurs when the frequency of the wave matches the frequency of electrons orbiting atomic nuclei in a material.
  • Reflected light bounces off a surface at the same wavelength with little or no change in energy.

Accommodation

Accommodation refers to the way the lenses inside our eyes accommodate for the fact that objects of interest may be close to or at a distance. Sharp images on the retina are the result of modifying the focal length of each lens.

  • The lens in each eye is located just behind the pupil. The shape of the lens is controlled by the ciliary muscle which forms a ring of flexible tissue around the edge of each lens.
  • The distance of objects of interest to an observer varies from infinity to next to nothing but the image distance between the centre of the lens and the retina always remains the same.
  • The focal length of a lens is the distance at which it brings parallel rays of light into focus on the retina.
    • A lens with a shorter focal length brings objects closer to the eye into focus and has a wider field of view.
    • Lens with a longer focal length bring objects at a greater distance from the eye into focus and has a narrower field of view.
  • Because the image distance is fixed in human eyes, they accommodate for this by using the ciliary muscle to alter the focal length of the lens.
    • To accommodate nearby objects the ciliary muscle contracts, making the lens more convex with a shorter focal length.
    • To accommodate distant objects the ciliary muscle relaxes, causing the lens to adopt a flatter and less convex shape with a longer focal length.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References
  • Accommodation refers to the way the lenses inside our eyes accommodate for the fact that objects of interest may be close to or at a distance. Sharp images on the retina are the result of modifying the focal length of each lens.
  • The focal length of a lens is the distance at which it brings parallel rays of light into focus on the retina.
    • A lens with a shorter focal length brings objects closer to the eye into focus and has a wider field of view.
    • Lens with a longer focal length bring objects at a greater distance from the eye into focus and has a narrower field of view.

Achromatic

Achromatic means without colour so refers to surfaces or objects that appear white, grey or black. Achromatic colours lack hue or saturation but can be described in terms of their brightness.

  • Near-neutral colours such as tints, shades or tones are often considered achromatic.
  • The lightest shades of pastel colours can be nearly achromatic, but they may still exhibit a subtle hint of hue.
  • Deep shadows and the world as it appears at night can appear achromatic, but may still retain some colour.
  • When mixing paint, achromatic colours are produced by adding black and/or white until the original colour is fully desaturated.
  • Achromatic colours are produced on digital screens by mixing red, green and blue light in equal proportions.
  • The RGB colour model produces achromatic colours when all three components of a colour have the same value. So the RGB colour values R = 128, G = 128, B = 128 produce a middle grey, which is an achromatic colour.
  • The way achromatic colours appear to an observer often depends on adjacent more saturated colours. So, next to a bright red couch, a grey wall may appear to have a slightly greenish tint due to the phenomenon known as simultaneous contrast.
Related diagrams

Each of the diagrams below can be viewed on its own page with a full explanation.

References
  • Achromatic means without colour so refers to surfaces or objects that appear white, grey or black. Achromatic colours lack hue or saturation but can be described in terms of their brightness.
  • When mixing paint, achromatic colours are produced by adding black and/or white until the original colour is fully desaturated.
  • Achromatic colours are produced on digital screens by mixing red, green and blue light in equal proportions.

Additive colour model

An additive colour model explains how different coloured lights (such as LEDs or beams of light) are mixed to produce other colours. The RGB colour model and HSB colour model are examples of additive colour models.

  • Additive colour refers to the methods used and effects produced by combining or mixing different wavelengths of light.
  • Additive colour models such as the RGB colour model and HSB colour model can produce vast ranges of colours by combining red, green, and blue lights in varying proportions.
  • An additive approach to colour is used to achieve precise control over the appearance of colours on digital screens of TVs, computers, and phones.
  • Subtractive colour models such as the CMY colour model provide methods for mixing pigments such as dyes, inks, or paints to produce different colours.
  • Both additive and subtractive colour models rely on mixing primary colours in different proportions:
    • CMY refers to the primary colours cyan (C), magenta (M) and yellow (Y).
    • CMYK refers to the primary colours cyan, magenta and yellow plus black (K).
    • RYB refers to the primary colours red (R), yellow (Y) and blue (B).
  • Both additive and subtractive colour models can be studied and understood by exploring colour wheels.
About additive & RGB colour

The RGB colour model used by TV, computer and phone screens involves additive colour mixing.

  • The RGB colour model produces all the colours seen by an observer on TV, computer and phone screens by creating arrays of red, green and blue pixels (picture elements) in different proportions.
  • Red, green and blue are called additive primary colours in an RGB colour model because just these three component colours alone can produce any conceivable colour if blended in the correct proportion.
  • Different colours are produced by varying the brightness of the component colours between completely off and fully on.
  • When fully saturated red, green and blue primary colours are mixed in equal amounts, they produce white.
  • A fully saturated hue is produced by a single wavelength (or narrow band of wavelengths) of light.
  • When any two fully saturated additive primary colours are mixed, they produce a secondary colour: yellow, cyan or magenta.
  • Some implementations of RGB colour models can produce millions of colours by varying the brightness of each of the three primary colours.
  • The additive RGB colour model cannot be used for mixing pigments such as paints, inks, dyes or powders.
  • The RGB colour model does not define the exact hue of the three primary colours so the choice of wavelengths for each primary colour is important if it is to be used as part of a colour-managed workflow.
  • The RGB colour model can be made device-independent by specifying a colour profile such as sRGB or Adobe RGB (1998) which ensures consistent results regardless of the device used to output an image.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References
  • An additive colour model explains how different coloured lights (such as LEDs or beams of light) are mixed to produce other colours.
  • Additive colour refers to the methods used and effects produced by combining or mixing different wavelengths of light.
  • The RGB colour model and HSB colour model are examples of additive colour models.
  • Additive colour models such as the RGB colour model and HSB colour model can produce vast ranges of colours by combining red, green, and blue lights in varying proportions.
  • An additive approach to colour is used to achieve precise control over the appearance of colours on digital screens of TVs, computers, and phones.

Adobe RGB colour space

The Adobe RGB (1998) colour space was developed by Adobe Systems. It aims to ensure the optimal range of colours available within the RGB colour model are accurately reproduced when output to a digital displays or printers.

  • The general purpose of a colour space is to determine the range of colours available within a specific workflow and may be determined by a user or programmatically.
  • The purpose of RGB (1998) was to improve on the gamut of colours that can be produced by the earlier sRGB colour space, primarily in the reproduction of cyan-green hues.
    • It can reproduce approximately 35% more colours compared to sRGB in ideal conditions.
    • It is estimated to cover approximately 50% to 70% of the colours perceived by the human eye in ideal conditions.
  • In a digital environment, the aim is to ensure that a selected range of colours appears consistent throughout a workflow and that the desired range of colours is successfully reproduced at the end of the process.
About colour spaces & examples
  • A colour space aims to accurately define the relationship between any selected colour within a colour model and how it will appear when it is reproduced by a specific device such as a digital display, printer or paint mixing machine.
  • When an artist selects a limited number of tubes of oil paint to add to a palette, they are already working within the RYB subtractive colour model and establishing the colour space in which they plan to work.
  • A colour space may aim to limit the number of colours or establish the widest possible gamut of reproducible colours.
  • Digital colour spaces are commonly used to accurately set the range of colours that can be output to and then displayed by digital screens and printers.
  • When a colour space is to be matched with a specific digital device such as a projector or printer, a colour profile is loaded along with the image file to ensure accurate colour reproduction.
  • A colour profile is a program that enables a piece of equipment, such as a digital printer, to know how to handle and process the information it receives, ensuring it can produce the intended colour output accurately.

Examples of colour spaces include:

RGB (Red, Green, Blue) Color Space
CMYK (Cyan, Magenta, Yellow, Key/Black) Color Space
  • CMYK is a subtractive colour model used in printing and design. It defines colours by subtracting varying amounts of cyan, magenta, yellow, and black ink from a white paper background. CMYK is used to achieve a wide range of colours on printed materials.
RYB (Red, Yellow, Blue) Color Space
  • RYB is an older subtractive colour model primarily used in traditional art and paint mixing. It consists of three primary colours: red, yellow, and blue. Mixing these colours creates secondary colours, such as orange, green, and violet. RYB is not used in modern digital design.
LAB Color Space
  • LAB is a device-independent colour space that represents colours in a way that is closer to human perception. It separates colour information into three channels: L (lightness), A (green to red), and B (blue to yellow). LAB is used in colour management and as an intermediate space when converting between different colour models.
HSB/HSV (Hue, Saturation, Brightness/Value) Color Space
  • HSB/HSV is a cylindrical colour model that represents colours based on three parameters: hue (the type of colour), saturation (the purity of the colour), and brightness/value (the intensity of the colour). It is often used in computer graphics and design software.
XYZ Color Space
  • XYZ is a CIE (Commission Internationale de l’Eclairage) standardized colour space that serves as a reference for defining other colour spaces. It is based on human vision and designed to be perceptually uniform. XYZ is used in various colour-related calculations and conversions.
Pantone Color Space
    • The Pantone colour system is widely used in the printing and design industries. It provides a standardized set of colours represented by specific codes. Each colour swatch is carefully defined to ensure consistency in printing and reproducing colours accurately.
Related diagrams

Each of the diagrams below can be viewed on its own page with a full explanation.

References
  • The general purpose of a colour space is to determine the range of colours available within a specific workflow and may be determined by a user or programmatically.
  • The Adobe RGB (1998) colour space  aims to ensure the optimal range of colours available within the RGB colour model are accurately reproduced when output to a digital displays or printers.
  • In a digital environment, the aim is to ensure that a selected range of colours appears consistent throughout a workflow and that the desired range of colours is successfully reproduced at the end of the process.

Airglow

Airglow is a faint, continual emission of light originating from the Earth’s upper atmosphere, typically between 80 and 400 kilometres in altitude. While often mistaken for distant starlight, it forms a distinct phenomenon with unique characteristics and scientific significance.

  • Emission Process: Airglow primarily results from chemiluminescence. Solar radiation ionizes atmospheric molecules like oxygen, nitrogen, and sodium during the day. These excited molecules later recombine with other particles at night, releasing energy as light in specific wavelengths.
  • Spectral Colours: Different molecules emit light at characteristic colours:
    • Green: Primary emission from excited oxygen atoms.
    • Red: Mainly from sodium atoms, contributing to the reddish band above the horizon.
    • Blue and violet: Emissions from hydroxyl (OH) and nitric oxide (NO) molecules.
  • Visibility and Variations: Airglow intensity varies due to altitude, wavelength, and location. Magnetic storms can enhance brightness, creating spectacular displays. Astronauts observe airglow as a luminous band encircling Earth.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References
  • 1
  • 2
  • Airglow is a faint, continual emission of light originating from the Earth’s upper atmosphere, typically between 80 and 400 kilometres in altitude. While often mistaken for distant starlight, it forms a distinct phenomenon with unique characteristics and scientific significance.
  • Emission Process: Airglow primarily results from chemiluminescence. Solar radiation ionizes atmospheric molecules like oxygen, nitrogen, and sodium during the day. These excited molecules later recombine with other particles at night, releasing energy as light in specific wavelengths.
  • Spectral Colours: Different molecules emit light at characteristic colours:
    • Green: Primary emission from excited oxygen atoms.
    • Red: Mainly from sodium atoms, contributing to the reddish band above the horizon.
    • Blue and violet: Emissions from hydroxyl (OH) and nitric oxide (NO) molecules.
  • Visibility and Variations: Airglow intensity varies due to altitude, wavelength, and location. Magnetic storms can enhance brightness, creating spectacular displays. Astronauts observe airglow as a luminous band encircling Earth.

Alexander’s band

Alexander’s band, also known as Alexander’s dark band, is an optical phenomenon observed in rainbows. It refers to the region between the primary and secondary bows, which often appears noticeably darker to an observer compared to the rest of the sky.

  • The areas of sky around a rainbow may appear blue or grey depending on weather conditions and cloud cover. However, these areas outside, inside, and between the primary and secondary rainbows tend to have distinct tonal differences from one another:
  • Alexander’s band can be explained by the fact that fewer photons are directed from this specific area of the sky toward an observer.
  • The raindrops that form a primary rainbow all direct exiting light downwards towards an observer so away from Alexander’s band.
  • The raindrops that form a secondary bow all direct light upwards, so away from Alexander’s band, before a second internal reflection directs light downwards towards an observer.
  • Alexander’s band is named after Alexander of Aphrodisias, an ancient Greek philosopher who made observations and comments about this phenomenon in his writings.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References
  • Alexander’s band, also known as Alexander’s dark band, is an optical phenomenon observed in rainbows. It refers to the region between the primary and secondary bows, which often appears noticeably darker to an observer compared to the rest of the sky.
  • The areas of sky around a rainbow may appear blue or grey depending on weather conditions and cloud cover. However, these areas outside, inside, and between the primary and secondary rainbows tend to have distinct tonal differences from one another:
    • The area inside the arcs of a primary rainbow always appears tonally lighter than the surrounding sky.
    • The area outside primary and secondary rainbows appears darker.
    • The area between primary and secondary rainbows appears the darkest – this is Alexander’s band.
  • Alexander’s band can be explained by the fact that fewer photons are directed from this specific area of the sky toward an observer.

Amacrine cells

Amacrine cells are interneurons in the human retina that interact with retinal ganglion cells and/or bipolar cells.

  • Amacrine cells are a type of interneuron within the human retina.
  • Amacrine cells are embedded in the retinal circuitry.
  • Amacrine cells are activated by and provide feedback to bipolar cells. They also form junctions with ganglion cells and communicate with each other.
  • Amacrine cells send complex spatial and temporal information about the visual world to ganglion cells.
  • Amacrine cells contribute additional information to the flow of data transmitted through bipolar cells and control and refine the response of ganglion cells (including their subtypes) to stimuli.
  • Most amacrine cells do not have long, tail-like axons. However, they do possess multiple connections to other neurons in their vicinity.
  • Axons are the part of neurons that transmit electrical impulses to other neurons.
  • Neurons, of which amacrine cells are an example, are the nerve cells that comprise the human central nervous system.
About amacrine cell functions

Amacrine cells are a type of neuron found in the retina, the light-sensitive tissue lining the back of the human eye. They play a critical role in the processing of visual signals before these signals are sent to the brain.

Amacrine cells are known to contribute to narrowly task-specific visual functions such as:

  • Spatial Contrast Enhancement: Amacrine cells contribute to a process called lateral inhibition, which helps to enhance the contrast between light and dark areas in a visual scene, thereby improving our ability to see edges and borders.
  • Temporal Contrast Enhancement: Amacrine cells play a role in detecting changes in light intensity over time, which helps us to perceive motion and changes in a visual scene.
  • Direction Selectivity: Certain types of amacrine cells are involved in detecting the direction of moving objects. These are known as starburst amacrine cells.
  • Centre-surround antagonism: Amacrine cells interact with both bipolar cells and retinal ganglion cells to contribute to the centre-surround antagonistic structure of ganglion cell receptive fields.
  • Complex Visual Processing: Amacrine cells form connections with multiple types of retinal cells, including bipolar cells and ganglion cells. This allows them to participate in complex processing and integration of visual information.
  • Inhibitory Signalling: Many amacrine cells arstae inhibitory interneurons, which means they can inhibit the activity of other neurons. This inhibitory function plays a role in shaping the output of retinal ganglion cells, which send visual information to the brain.
  • Regulation of Circadian Rhythm: Some amacrine cells release a pigment called melanopsin and are involved in non-image-forming visual functions, such as the regulation of circadian rhythms and the pupillary light reflex.
  • Neurotransmitter Release: Amacrine cells can release a variety of neurotransmitters, including GABA, glycine, dopamine, and others, allowing them to modulate the activity of various neural circuits in the retina.
About centre-surround antagonism

Centre-surround antagonism refers to the way retinal neurons organize their receptive fields.

  • Centre-surround antagonism refers to the way that light striking the human retina is processed by groups of light-sensitive cone cells.
  • The centre component is primed to measure the sum-total of signals received from a small number of cone cells directly connected to a bipolar cell.
  • The surround component is primed to measure the sum of signals received from a much larger number of cones around the centre point.
  • The two signals are then compared to find the degree to which they disagree.
Related diagrams

Each of the diagrams below can be viewed on its own page with a full explanation.

References
  • Amacrine cells are interneurons in the human retina within the eyeball that interact with retinal ganglion cells and/or bipolar cells.
  • Amacrine cells are an example of neurons which are the nerve cells that comprise the human central nervous system.
  • Amacrine cells are activated by and provide feedback to bipolar cells. They also form junctions with ganglion cells and communicate with each other.
  • Amacrine cells send complex spatial and temporal information about the visual world to ganglion cells.

Amplitude

The amplitude of an electromagnetic wave is directly connected with the amount of energy it carries.

In a wave diagram, amplitude is represented as the distance from the center line (or midpoint) of a wave to the top of a crest or to the bottom of a corresponding trough.

  • When the amplitude of an electromagnetic wave increases, the overall distance between any peak and the next trough also increases.
  • The greater the amplitude of a wave, the more energy it carries.
  • The quantity of energy carried by an electromagnetic wave is proportional to the amplitude squared.
  • The amplitude of the electric field of an electromagnetic wave is measured in volts per meter (V/m), while the amplitude of the magnetic field is measured in amperes per meter (A/m).
  • Amplitude has an indirect correlation with the perception of the intensity of light and the brightness of colour as perceived by an observer because additional factors such as phase and interference must be taken into account.
About amplitude, brightness, colour brightness and intensity

The terms amplitude, brightness, colour brightness and intensity are easily confused. In this resource:

Amplitude
Brightness
  • Brightness refers to a property of light, to how strong a light source or light reflected off an object appears to be.
  • Brightness is related to how things appear from the point of view of an observer.
    • When something appears bright it seems to radiate or reflect more light or colour than something else.
    • Brightness may refer to a light source, an object, a surface, transparent or translucent medium.
    • The brightness of light depends on the intensity or the amount of light an object emits( eg. the Sun or a lightbulb).
    • The brightness of the colour of an object or surface depends on the intensity of light that falls on it and the amount it reflects.
    • The brightness of the colour of a transparent or translucent medium depends on the intensity of light that falls on it and the amount it transmits.
    • Because brightness is related to intensity, it is related to the amplitude of electromagnetic waves.
    • Brightness is influenced by the way the human eye responds to the colours associated with different wavelengths of light. For example, yellow appears relatively brighter than reds or blues to an observer.
Colour Brightness

So colour brightness can refer to the difference between how a colour appears to an observer in well-lit conditions and its subdued appearance when in shadow or when poorly illuminated.

Intensity
    • Intensity refers to the amount of light produced by a light source or the amount of light that falls on a particular area of the object.
    • So intensity measures the energy carried by a light wave or stream of photons:
      • When light is modelled as a wave, intensity is directly related to amplitude.
      • When light is modelled as a particle, intensity is directly related to the number of photons present at any given point in time.
      • Light intensity falls exponentially as the distance from a point light source increases.
      • Light intensity at any given distance from a light source is directly related to its power per unit area (when the area is measured on a plane perpendicular to the direction of propagation of light).
      • The power of a light source describes the rate at which light energy is emitted and is measured in watts.
      • The intensity of light is measured in watts per square meter (W/m2).
      • Cameras use a light meter to measure the light intensity within an environment or reflected off a surface.
Related diagrams

Each of the diagrams below can be viewed on its own page with a full explanation.

References
  • The amplitude of an electromagnetic wave is directly connected with the amount of energy it carries.
  • In a wave diagram, amplitude is represented as the distance from the center line (or midpoint) of a wave to the top of a crest or to the bottom of a corresponding trough.
  • When the amplitude of an electromagnetic wave increases, the overall distance between any peak and the next trough also increases.
  • The quantity of energy carried by an electromagnetic wave is proportional to the amplitude squared.
  • Amplitude has an indirect correlation with the perception of the intensity of light and the brightness of colour as perceived by an observer because additional factors such as phase and interference must be taken into account.

Analogous colours

Analogous colours are colours that are very similar to one another and appear next to each other on a colour wheel.

  • Analogous colours are colours with similar hues.
  • An example of a set of analogous colours is red, reddish-orange, orange, and yellow-orange.
  • An analogous colour scheme creates a rich appearance but is generally less vibrant than a colour scheme with contrasting colours.
  • Increasing the number of segments on a colour wheel shows analogous colours more clearly because the gradation between adjacent hues becomes finer.
Related diagrams

Each of the diagrams below can be viewed on its own page with a full explanation.

References
  • Analogous colours are colours that are very similar to one another and appear next to each other on a colour wheel.
  • Analogous colours are colours with similar hues.
  • An example of a set of analogous colours is red, reddish-orange, orange, and yellow-orange.
  • An analogous colour scheme creates a rich appearance but is generally less vibrant than a colour scheme with contrasting colours.
  • Increasing the number of segments on a colour wheel shows analogous colours more clearly because the gradation between adjacent hues becomes finer.

Angle of deflection

When discussing the formation of rainbows, the angle of deflection measures the angle between the initial path of a light ray before it hits a raindrop, and the angle of deviation, which measures how much the ray bends back on itself in the course of refraction and reflection towards an observer.

  • The angle of deflection and the angle of deviation are always directly related to one another and together add up to 180 degrees.
  • The angle of deflection equals 180 degrees minus the angle of deviation. So, it’s clear the angle of deviation is always equal to 180 degrees minus the angle of deflection.
  • In any particular case, the angle of deflection is always the same as the viewing angle because the incident rays of light that form a rainbow all follow paths that run parallel with the rainbow axis.
Related Points
  • Any ray of light (stream of photons) travelling through empty space, unaffected by gravity, travels in a straight line forever.
  • When light travels from a vacuum or from one transparent medium into another, it undergoes refraction causing it to change both direction and speed.
  • The more a light ray changes direction when it passes through a raindrop, the smaller the angle of deflection will be.
  • It is the optical properties of raindrops that determine the angle of deflection of light as it exits a raindrop.
  • Because of the optical properties of primary rainbows, no rays of light within the visible part of the electromagnetic spectrum exit a raindrop at an angle of deflection larger than 42.4 degrees.
  • Because of the optical properties of secondary rainbows, no rays of light within the visible part of the electromagnetic spectrum exit a raindrop at an angle of deflection larger than 50.4 degrees.
Implications of the Angle of Deflection for Raindrops
  •  For a single incident ray of light of a known wavelength striking a raindrop at a known angle:
    • To appear in a primary rainbow it cannot exceed an angle of deflection of more than 42.4 degrees. This corresponds with the minimum angle of deviation.
    • 42.40 is the angle of deflection that results in red appearing along the outside edge of a primary rainbow from the point of view of an observer.
    • 180 degrees minus 137.6 degrees equals 42.4 degrees, which is the biggest angle of deflection for any visible light ray if it is to appear within a primary rainbow.
    • 180 degrees minus 139.3 degrees equals 40.7 degrees, which is the angle of deflection for a light ray that appears violet and forms the inside edge of a primary rainbow.
    • Angles of deviation between 137.6 degrees and 139.3 degrees match viewing angles and angles of deflection between 42.4 degrees (red) and 40.7 degrees (violet).
    • An angle of deviation of 137.6 degrees (so viewing angles of 42.4 degrees) matches the appearance of red light with a wavelength of about 720 nm.
  • The range of angles of deflection that create the impression of colour for an observer is not related to droplet size.
  • The laws of refraction (Snell’s law) and reflection can be used to work out the angle of deviation of white light in a raindrop.
  • The angle of deviation can be fine-tuned for any specific wavelength by making small adjustments to the refractive index.
About rainbows, raindrops & angles
How they interconnect
    • In any particular example of a ray of light passing through a raindrop, the angle of deviation and the angle of deflection are directly related to one another and together add up to 1800.
    • The angle of deviation is always equal to 1800 minus the angle of deflection. So clearly the angle of deflection is always equal to 1800 minus the angle of deviation.
    • In any particular example, the angle of deflection is always the same as the viewing angle because the incident rays of light that form a rainbow are all approaching on a trajectory running parallel to the rainbow axis.
Related diagrams

Each of the diagrams below can be viewed on its own page with a full explanation.

References
  • When discussing the formation of rainbows, the angle of deflection measures the angle between the initial path of a light ray before it hits a raindrop, and the angle of deviation, which measures how much the ray bends back on itself in the course of refraction and reflection towards an observer.
  • See this diagram for an explanation: Rainbow anatomy
  • The angle of deflection and the angle of deviation are always directly related to one another and together add up to 180 degrees.
  • The angle of deflection equals 180 degrees minus the angle of deviation. So, it’s clear the angle of deviation is always equal to 180 degrees minus the angle of deflection.
  • In any particular case, the angle of deflection is always the same as the viewing angle because the incident rays of light that form a rainbow all follow paths that run parallel with the rainbow axis.

Angle of incidence

The angle of incidence measures the angle at which incoming light strikes a surface.

  • When light is travelling towards something it is said to be incident to that surface or object.
  • The angle of incidence is measured between a ray of incoming light and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
  • Expressed more formally, in optics, the normal is a geometric construct, a line drawn perpendicular to the interface between two media at the point of contact. This conceptually defined reference line is crucial for characterizing various light-matter interactions, such as reflection, refraction, and absorption.
  • Incident light may have travelled from the Sun or a man-made source or may have already been reflected off another surface such as a mirror.
  • When incident light strikes a surface or object it may undergo absorption, reflection, refraction, transmission or any combination of these optical effects.
About lines that are normal to one another
  • If one line is normal to another, then it is at right angles.
  • In geometry, a normal (or the normal) refers to a line drawn perpendicular to and intersecting another line, plane or surface.
  • In the field of optics, the normal is a line drawn on a ray-tracing diagram perpendicular to (at 900 to), the boundary between two media.
  • If the boundary between two media is curved then the normal is drawn at a tangent to the boundary.
Related diagrams

Each of the diagrams below can be viewed on its own page with a full explanation.

References
  • The angle of incidence measures the angle at which incoming light strikes a surface.
  • The angle of incidence is measured between a ray of incoming light and an imaginary line called the normal.
  • See this diagram for an explanation: Reflection of a ray of light
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
  • If the boundary between the media is curved, then the normal is drawn at a tangent to the boundary.

Angle of reflection

The angle of reflection is the angle between the incident light ray and the reflected light ray, both measured from an imaginary line called the normal.

  • According to the law of reflection, the angle of incidence (the angle between the incident ray and the normal) is always equal to the angle of reflection.
  • The angle of reflection is measured between the reflected ray of light and an imaginary line perpendicular to the surface, known as the normal.
  • In optics, the normal is a straight line drawn on a ray-tracing diagram at a 90º angle (perpendicular) to the boundary where two different media meet.
  • Expressed more formally, in optics, the normal is a geometric construct, a line drawn perpendicular to the interface between two media at the point of contact. This conceptually defined reference line is crucial for characterizing various light-matter interactions, such as reflection, refraction, and absorption.
  • If the boundary between two media is curved, the normal is drawn perpendicular to the tangent to that point on the boundary.
  • Reflection can be diffuse (when light reflects off rough surfaces) or specular (in the case of smooth, shiny surfaces), affecting the direction of reflected rays.
About the laws of reflection

When light reflects off a surface or object, it behaves predictably, following three laws known as the laws of reflection.

  • The three laws of reflection are:
    • The incident ray, the reflected ray, and the normal to the surface all lie within the same plane.
    • The angle of incidence, which is the angle between the incident ray and the normal, is equal to the angle of reflection, which is the angle between the reflected ray and the normal.
    •  The incident ray and the reflected ray are always on opposite sides of the normal, and all three lie within the same plane.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

https://en.wikipedia.org/wiki/Reflection_(physics)

  • The angle of reflection measures the angle at which reflected light bounces off a surface.
  • The angle of reflection is measured between a ray of light which has been reflected off a surface and an imaginary line called the normal.
  • See this diagram for an explanation: Reflection of a ray of light
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
  • If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.

Angle of refraction

The angle of refraction indicates the extent to which light bends and changes direction as it passes from one transparent medium into another.

  • The angle of refraction is measured between the refracted ray of light and the normal.
  • The normal is an imaginary line drawn on a ray-tracing diagram perpendicular to the surface at the point of incidence.
  • Expressed more formally, in optics, the normal is a geometric construct, a line drawn perpendicular to the interface between two media at the point of contact. This conceptually defined reference line is crucial for characterizing various light-matter interactions, such as reflection, refraction, and absorption.
  • The equation used to determine the angle of refraction is often referred to as Snell’s law or the law of refraction.
  • The law of refraction describes the relationship between the angle of incidence and the angle of refraction when light crosses the boundary between two transparent media, such as from air to water.
  • In the field of optics, Snell’s law is applied when drawing a ray-tracing diagram to calculate the angles of incidence and refraction and is also used experimentally to determine the refractive index of a given medium.
  • The refractive index of a medium can significantly affect the degree to which light is refracted when entering that medium.
  • For instance, light is refracted more when it enters a medium with a high refractive index, such as glass, than when it enters a medium with a low refractive index, like air.
  • Total internal reflection is a phenomenon that takes place when light attempts to pass from a medium with a higher refractive index to one with a lower refractive index at an angle greater than the so-called “critical angle”. The outcome is that all of the light is reflected back into the first medium instead of being refracted.
About lines that are normal to one another
  • If one line is normal to another, then it is at right angles.
  • In geometry, a normal (or the normal) refers to a line drawn perpendicular to and intersecting another line, plane or surface.
  • In the field of optics, the normal is a line drawn on a ray-tracing diagram perpendicular to (at 900 to), the boundary between two media.
  • If the boundary between two media is curved then the normal is drawn at a tangent to the boundary.
Related diagrams

Each of the diagrams below can be viewed on its own page with a full explanation.

References

The angle of refraction measures the angle to which light bends as it passes across the boundary between different media.

  • The angle of refraction is measured between a ray of light and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
  • See this diagram for an explanation: Refraction of a ray of light
  • If the boundary between the media is curved, the normal is drawn perpendicular to the boundary.

Angular distance

When discussing rainbows, angular distance is the angle between the line from the observer to the centre of the rainbow (rainbow axis) and the line from the observer to a specific colour within the arc of a rainbow.

  • Angular distance, viewing angle, and angle of deflection are related concepts that produce similar values, expressed in degrees.
  • Angular distance is one of the angles measured on a ray-tracing diagram that illustrates the sun, an observer, and a rainbow from a side view.
  • Think of angular distance as the angle between the line to the centre of a rainbow down which an observer looks and the line to a specific colour in its arc. The red light is deviated by about 42.4° and violet light by about 40.7°.
  • Angular distances for different colours are fairly constant, determined by the laws of refraction and reflection inside the water droplets.
  • The sun’s position, the observer’s location, and the specific location of rainfall all influence where a rainbow will appear from an observer’s perspective.
  • The coloured arcs of a rainbow form parts of circles (discs or cones), and all these circles share a common centre—the observer’s anti-solar point, which is directly opposite the sun.
  • The angular distance to a specific colour is consistent, regardless of the point chosen along the arc of that colour in the rainbow.
  • The angular distance for any observed colour in a primary rainbow varies from approximately 42.4° for red to 40.7° for violet.
  • The angular distance for any observed colour in a secondary rainbow ranges from about 53.4° for red to 50.4° for violet, measured from the antisolar point.
  • The angular distance for any specific colour within a rainbow can indeed be calculated, depending on its wavelength and how it refracts within the water droplets.
  • From an observer’s viewpoint, all the incoming light rays seen by the observer appear parallel to each other as they approach a raindrop.
  • Most of the observable incident rays that strike a raindrop follow paths that place them outside the range of possible viewing angles. The unobserved rays are all deflected towards the centre of a rainbow.
  • Many of the light rays that strike a raindrop follow paths that place them outside the range of possible viewing angles, so do contribute to the coloured arcs seen by an observer.
About viewing angles, angular distance and angles of deflection
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References
  • When discussing rainbows, angular distance is the angle between the line from the observer to the centre of the rainbow (rainbow axis) and the line from the observer to a specific colour within the arc of a rainbow.
  • See this diagram for an explanation: Angular distance & Raindrop colour
  • Angular distance is one of the angles measured on a ray-tracing diagram that illustrates the sun, an observer, and a rainbow from a side view.
  • Think of angular distance as the angle between the line to the centre of a rainbow down which an observer looks and the line to a specific colour in its arc. The red light is deviated by about 42.4° and violet light by about 40.7°.

Artificial light source

An artificial light source is any source of light created by humans, as opposed to natural light sources like the sun or stars. Artificial light sources are generated by converting different forms of energy into light.

There are several major categories of artificial light sources such as:

  • Incandescent: These work by heating a filament until it glows, emitting light (traditional light bulbs).
  • Fluorescent: Electric current triggers gas inside the bulb to produce ultraviolet light, which a phosphor coating converts into visible light.
  • LED (Light-Emitting Diode): Electricity excites semiconductors, causing them to emit light.
  • Gas-discharge lamps: Electric current passes through a gas, producing bright light (e.g., neon signs, street lamps).
Common Examples
  • Light bulbs: Incandescent, fluorescent (CFLs), and LED bulbs in our homes.
  • Street lights and car headlights
  • Flashlights and torches
  • Digital screens: Electronics like phones, TVs, and computer monitors that use light for display.
Key Points
  • Control: Artificial light sources give us the ability to control lighting conditions, extending our active hours and providing illumination for work and leisure.
  • Efficiency: Advancements in artificial lighting have focused on energy efficiency, such as the widespread adoption of LED lights.
  • Impact: Artificial light sources can have impacts on the environment and human health, such as light pollution and disruption of circadian rhythms.
Light sources
Emission mechanism DescriptionExamples
LIGHT-EMITTING PROCESS
LuminescenceLight emission due to the excitation of electrons in a material.Electrons within a material gain energy and then release light as they return to a lower energy state.Bioelectroluminescence
Electroluminescence
Photoluminescence
- Fluorescence
- Phosphorescence
Sonoluminescence
Thermoluminescence
Blackbody radiation (Type of thermal radiation)Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.All objects above temperature of absolute zero.
ChemiluminescenceLight from natural and artificial chemical reactions.Light from natural and artificial chemical reactions.Bioluminescence
Chemiluminescent reactions:
- Luminol reactions
- Ruthenium chemiluminescence
Nuclear reactionLight emission as a byproduct of nuclear reactions (fusion or fission).Light emitted as a byproduct of nuclear reactions.Nuclear reactors
Stars undergoing fusion
Thermal radiationLight emission due to the thermal excitation of atoms and molecules at high temperatures.Light emission due to the thermal excitation of atoms and molecules.Sun
Stars
Incandescent light bulbs
TriboluminescenceLight emission due to mechanical stress applied to a material.Light emission due to the mechanical stress applied to a material, causing the movement of electric charges and subsequent light emission.Sugar crystals cracking
Adhesive tape peeling
Quartz crystals fracturing.
Natural light source
Fireflies
Deep-sea creatures
Glowing mushrooms		Bioluminescence Light emission from biological organisms.Involves the luciferase enzyme.
Sun
Stars		Nuclear FusionLight emission as a byproduct of nuclear fusion reactions in stars.Electromagnetic spectrum (visible light, infrared, ultraviolet).
Fire
Candles		Thermal radiationLight emission due to the thermal excitation of atoms and molecules during the combustion of a fuel source.Burning of a fuel source, releasing heat and light.
Artificial light source
Fluorescent lights Highlighters
Safety vests		Chemiluminescence Light emission from chemical reactions.Fluorescence (absorption and re-emission of light).
Glow sticks
Emergency signs		ChemiluminescenceLight emission due to phosphorescence - a type of chemiluminescence.A type of chemiluminescence where light emission is delayed after the initial excitation.
Glow sticks
Light sticks		Chemiluminescence Chemiluminescence Light emission from a chemical reaction that does not involve combustion.
Tungsten light bulbs
Toasters		Thermal radiationHeated filament radiates light and heat.Light emission from a hot filament.
Fluorescent lamps
LED lights		ElectroluminescenceExcitation of atoms by electric current.Light emission when electric current excites atoms in a material.
Neon signsElectrical DischargeDischarge of electricity through gas.Light emission when electricity flows through a gas.
Sugar crystals cracking
Pressure-sensitive adhesives		TriboluminescenceLight emission from friction or pressure.Light emission due to mechanical forces.
Fluorescent paint Highlighters
Safety vests		PhotoluminescenceAbsorption and subsequent re-emission of light at a lower energy.Absorption and re-emission of light.

Light Sources: Mechanism, examples, and everyday applications

Footnote: Cerenkov radiation and Synchrotron radiation are not included in the table because they are not conventionally classified as light sources.

Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References
  • An artificial light source is any source of light created by humans, as opposed to natural light sources like the sun or stars. Artificial light sources are generated by converting different forms of energy into light.
  • There are several major categories of artificial light sources such as:
    • Incandescent: These work by heating a filament until it glows, emitting light (traditional light bulbs).
    • Fluorescent: Electric current triggers gas inside the bulb to produce ultraviolet light, which a phosphor coating converts into visible light.
    • LED (Light-Emitting Diode): Electricity excites semiconductors, causing them to emit light.
    • Gas-discharge lamps: Electric current passes through a gas, producing bright light (e.g., neon signs, street lamps).

Atmospheric rainbow

An atmospheric rainbow is an arc or circle of spectral colours and appears in the sky when an observer is in the presence of strong sunshine and rain.

  • Atmospheric rainbows:
    • Are caused by sunlight reflecting, refracting and dispersing inside raindrops before being seen by an observer.
    • Appear in the section of the sky directly opposite the Sun from the point of view of an observer.
    • Become visible when millions of raindrops reproduce the same optical effects.
  • Atmospheric rainbows often appear as a shower of rain is approaching, or has just passed over. The falling raindrops form a curtain on which sunlight falls.
  • To see an atmospheric rainbow, the rain must be in front of the observer and the Sun must be in the opposite direction, at their back.
  • A rainbow can form a complete circle when seen from a plane, but from the ground, an observer usually sees the upper half of the circle with the sky as a backdrop.
  • Rainbows are curved because light is reflected, refracted and dispersed symmetrically around their centre-point.
  • The centre-point of a rainbow is sometimes called the anti-solar point. ‘Anti’, because it is opposite the Sun with respect to the observer.
  • An imaginary straight line can always be drawn that passes through the Sun, the eyes of an observer and the anti-solar point – the geometric centre of a rainbow.
  • A section of a rainbow can easily disappear if anything gets in the way and forms a shadow. Hills, trees, buildings and even the shadow of an observer can cause a portion of a rainbow to vanish.
  • Not all rainbows are ‘atmospheric’. They can be produced by waterfalls, lawn sprinklers and anything else that creates a fine spray of water droplets in the right conditions.

Atom

An atom is the smallest unit of a chemical element that retains all of its physical and chemical properties.

  • At the core of an atom is a nucleus that contains protons which are positively charged sub-atomic particles. The number of protons defines the atomic number and thus the chemical element of the atom. For instance, a hydrogen atom has one proton.
  • In addition to protons, the nucleus of an atom also houses neutrons, sub-atomic particles with a mass slightly larger than protons but with no electrical charge.
  • Circling the nucleus are negatively charged particles called electrons, which are kept in place by their attraction to the positively charged protons in the nucleus.
  • In a neutral atom, the quantity of electrons equals the number of protons.
  • A neutral atom is an atom where the number of protons, which are positively charged, equals the number of electrons, which are negatively charged.
  • When an electron is gained or lost from an atom, it forms a charged particle known as an ion.
    • A cation is a positively charged ion resulting from the loss of electrons, making the number of electrons fewer than the protons.
    • An anion is a negatively charged ion resulting from the gain of electrons, making the number of electrons more than the protons.
  • The total number of protons and neutrons in an atom’s nucleus determines its atomic mass.
  • Atomic mass measures the total mass of protons and neutrons in an atom and helps in the arrangement of elements in the periodic table.
  • In atomic theory and quantum mechanics, an atomic orbital is a mathematical function that describes the behaviour and location of an electron in an atom’s electron cloud.
Related diagrams

References
  • An atom is the smallest unit of a chemical element that retains all of its physical and chemical properties.
  • At the core of an atom is a nucleus that contains protons which are positively charged sub-atomic particles. The number of protons defines the atomic number and thus the chemical element of the atom. For instance, a hydrogen atom has one proton.
  • In addition to protons, the nucleus of an atom also houses neutrons, sub-atomic particles with a mass slightly larger than protons but with no electrical charge.
  • Circling the nucleus are negatively charged particles called electrons, which are kept in place by their attraction to the positively charged protons in the nucleus.

Atom as a light source

An atom isn’t typically considered a light source in the conventional sense because it can’t generate light spontaneously. In its “ground state,” an atom is stable and doesn’t emit light.

  • Atoms however do play a crucial role in the absorption and emission of light and in interactions such as light scattering when external energy allows them to reach an excited, “activated” state, where they release energy as light.
  • This energy can come from various sources such as:
    • Incandescent bulbs need thermal energy (heat) to excite atoms in a filament so they emit light.
    • Electrons in LEDs need an electric field to excite them to the point where they emit light.
    • Chemical reactions provide the energy to enable electrons to reach an excited state that results in chemiluminescence.
  • Depending on the energy source and atom arrangement, different mechanisms are involved in light emission such as electron transitions and molecular vibrations.
Atomic luminescence, how it works
  • Tiny atoms can produce brilliant light. Light emission, observed in diverse contexts like fireflies, lightbulbs, and even rainbows, share a connection to the microscopic world of atoms. Despite their minuscule size, atoms possess the ability to generate light through specific physical mechanisms.
  • This page looks into the underlying principles of atomic luminescence, focusing on the role of electrons within atomic structure. It examines how an atom’s energy levels, interactions with external stimuli, and transitions between these levels contribute to the emission of light.Atomic luminescence refers to the phenomenon where individual atoms emit light. Despite their minuscule size, atoms possess the ability to generate visible light through specific physical mechanisms. This ability arises from the behaviour of electrons within the atom, particularly their interactions with energy and their transitions between different energy levels.Imagine the atom as a central nucleus orbited by electrons. These electrons exist in specific energy levels, often referred to as shells and orbitals. When an external stimulus, like heat, light,  an electric field or a chemical reaction interacts with an atom, it can excite an electron, boosting its energy level. However, atoms are unstable in these excited states. To return to their stable ground state, they release the excess energy, often in the form of light. This release of energy as light is the essence of atomic luminescence.The specifics of the mechanisms governing this release determine the characteristics of the emitted light, such as its colour, intensity, and duration. Different mechanisms like thermal excitation (incandescent bulbs), chemical reactions (glow sticks), and electrical fields (LEDs) offer pathways for atoms to convert energy into the phenomenon of light.
  • Here are some examples of the mechanisms that produce light:
  • Thermal Emission: This process involves the excitation of electrons within an atom due to heat. As the atom’s temperature rises, electrons absorb thermal energy, transitioning to higher energy levels. Upon returning to their ground state, they release this excess energy in the form of light, as evident in incandescent bulbs and the light produced by stars.Chemiluminescence: This mechanism triggers light emission through chemical reactions. Specific chemical reactions release energy, directly promoting excited electron states within participating molecules. These then return to their ground state, emitting light, as showcased in glow sticks and by certain biological organisms.Electroluminescence: Here, electrical fields excite electrons within a material’s atomic structure. This energy boost enables electron transitions, leading to light emission by light-emitting diodes (LEDs) and various displays.Fluorescence and Phosphorescence: Both mechanisms involve the absorption of light by atoms, followed by its re-emission at different wavelengths and timescales. While both involve excited electron states, fluorescence exhibits a prompt return to the ground state, leading to immediate light emission. Phosphorescence, however, involves a delayed emission.

Unveiling the Atom

  • The basic structure of an atom, highlighting the nucleus and electrons.
  • The concept of energy levels and how excited electrons play a crucial role in light emission.

The Electron’s Role: properties and behaviour within the atom

  • Charge and Mass: Emphasize the electron’s negative charge and tiny mass compared to the nucleus.
  • At the heart of every atom lies a miniature solar system of sorts, with a central nucleus orbited by even tinier particles called electrons. While these electrons might seem insignificant due to their minuscule size, their unique properties, namely their negative charge and extremely small mass, play a crucial role in shaping the atom’s behaviour and ultimately, the world around us.
    • Negative Charge: Unlike the positively charged protons within the nucleus, electrons carry a negative electrical charge. Think of them as tiny magnets with a specific “polarity” opposite to protons. This inherent charge is the foundation for various interactions, including attracting positively charged particles and repelling other electrons, ultimately influencing how atoms bond with each other to form molecules and materials.
    • Tiny Mass: Compared to the nucleus, an electron’s mass is truly minuscule. Imagine the nucleus as a basketball and the electron as a grain of sand – that’s roughly the size difference! This small mass gives electrons a remarkable degree of agility, allowing them to move much faster and respond more readily to external stimuli like light or electrical fields.

    The Significance of Smallness:

    • Energy Levels: Due to their small mass and the influence of the electromagnetic force, electrons exist in specific energy levels around the nucleus. Imagine these levels as shells or orbits, each with a defined energy. Electrons can “jump” between levels by absorbing or releasing energy, often in the form of light. This phenomenon plays a vital role in understanding light emission (like in LEDs) and absorption (like in photosynthesis).
    • Chemical Bonds: The electrical attraction between positively charged nuclei and negatively charged electrons forms the basis of chemical bonds. Different arrangements of electrons in these orbitals determine how atoms interact and create the diverse array of molecules and materials we see in the world.

    Understanding the Contrast:

    The vast difference in charge and mass between electrons and the nucleus plays a critical role in their behaviour:

    • Nucleus: The massive and positively charged nucleus acts as the central powerhouse, holding the atom together and defining its identity. However, due to its large mass, it moves comparatively slower and doesn’t readily participate in energy transitions involving light.
    • Electrons: Their small mass and opposite charge make them agile dancers around the nucleus. Their movements and energy transitions are crucial for understanding how atoms interact with light and each other, shaping the chemical and physical properties of matter.

Energy Levels: Explain how electrons occupy discrete energy levels around the nucleus, each level corresponding to a specific energy state.

Quantum Jumps: Describe how electrons can absorb energy (photons) to jump to higher energy levels and release energy (photons) as they return to lower levels, emitting light in the process.

Electron Configurations: Briefly touch upon the concept of electron configurations and how they influence an atom’s ability to emit light.

Pathways to Radiance

  • Different mechanisms by which atoms emit light:
    • Thermal Emission: Explain how heat excites electrons, prompting them to release photons as they return to lower energy levels (incandescent bulbs).
    • Chemiluminescence: Describe how chemical reactions can directly trigger electron transitions and light emission (glow sticks).
    • Electroluminescence: Explain how applying electricity excites electrons, leading to light emission (LEDs).
    • Fluorescence and Phosphorescence: Describe how atoms can absorb and then re-emit light at different wavelengths and timescales.

Nuclear Transitions

  • While nuclear fusion fuels the Sun’s brilliance, a different phenomenon involving the nucleus itself can emit light within atoms.
    • Nuclear Decay: Radioactive atoms can undergo spontaneous transformations where their unstable nuclei emit various particles, including energetic photons (gamma rays) as part of the process. Imagine an excited nucleus “calming down” by releasing this excess energy as light. This phenomenon finds applications in medical imaging and dating ancient objects.
    • Positron Emission: Remember the positrons mentioned in the Sun’s fusion process? In certain cases of radioactive decay, an unstable nucleus transforms a proton into a neutron, releasing a positron (essentially an anti-electron) alongside a neutrino. When this positron encounters an electron, they “annihilate,” and convert their combined mass into pure energy, often in the form of two gamma rays travelling in opposite directions.
    • Synchrotron Radiation: This phenomenon doesn’t occur within individual atoms but involves their charged constituents. Imagine accelerating tiny subatomic particles like electrons to near the speed of light in circular paths using powerful magnets. As these charged particles change direction, they emit electromagnetic radiation across a broad spectrum, from infrared to X-rays. This forms the basis of synchrotrons, facilities used for research in various fields like materials science and medicine.
    • Cherenkov Radiation: Similar to synchrotron radiation, this phenomenon involves charged particles (often electrons) travelling faster than the speed of light within a specific medium (like water or glass). Their interaction with the medium disrupts the electromagnetic field, creating a faint, directional glow known as Cherenkov radiation. This has applications in particle physics and even detecting cosmic rays.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References
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Summary