Dictionary category: Definitions

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.
• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

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.

• 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.

Attributes of visual perception associated with the response of the human eye and brain to light include:

• Brightness discrimination: The capacity to discern variations in illumination within a visual scene.
• Colour constancy is the ability to perceive colours as relatively constant, even under changing lighting conditions.
• Colour perception: The capability to discern differences in colour in the presence of light, which includes all shades of grey between black and white.
• Depth perception: The ability to estimate the distance between oneself and objects, and also the distance between separate objects.
• Figure-ground perception: The ability to distinguish significant details from a complex background.
• Form constancy: The capability to identify a shape as constant, even when it changes in size or orientation.
• Rapid visual processing: The capacity for swiftly and accurately identifying visual details in rapidly changing objects and surroundings.
• Sensory processing: The accurate detection, interpretation, and response to visual information, integrated with other sensory experiences.
• Spatial relationships: The capacity to comprehend the relative positioning of objects, including their distance, direction of movement, and location with respect to the observer.
• Visual attention: The skill to concentrate on relevant visual information while disregarding insignificant background details.
• Visual closure: The capacity to identify a shape or object even when part of it is obscured or absent.
• Visual discrimination: The capability to perceive differences or similarities in objects, based on attributes such as size, colour, shape, and so on.
• Visual memory: The capacity to recall the appearance of a shape or object.
• Visual-motor integration: The ability to coordinate vision and hand movement to perform tasks such as catching a ball or writing.
• Visual sequential memory: The capacity to recall the sequence of items or events.
• Visual tracking: The ability to efficiently move one’s gaze from one object to another.

• Auroras are caused by the interaction of charged particles (such as electrons), ejected from the Sun (solar wind), with the Earth’s magnetosphere.
• The magnetosphere accelerates electrons as they enter the atmosphere after travelling from the Sun.
• The colour and pattern of an aurora are partly determined by the degree of acceleration given to the particles as they enter the atmosphere.
• Different gases in Earth’s atmosphere produce different colours of auroras when struck by solar particles. Oxygen produces green and red light, while nitrogen gives blue and purple.
• The shape of an aurora depends on the Earth’s magnetic field lines, as the charged particles travel along these lines.
• The visibility of auroras depends not only on geographical location and time of day but also on solar activity. Stronger solar winds typically cause more intense auroras.