Illuminance

Illuminance refers to the amount of light from a natural or artificial light source that falls on a surface. It is usually used to describe the usable light, regardless of the total brightness of the light source.

  • When a book is placed on a table, different levels of illuminance can be observed depending on whether the sky is overcast, the time of day, or whether the surface is indirectly lit.
  • Illuminance is a measure of the amount of light that falls on a surface per unit area. It is determined by the intensity of the light source and the distance from the light source to the surface, but is independent of the characteristics of the surface it strikes, such as its colour or reflectivity.
  • Illuminance is a measure of the quantity of light that falls on a surface, but it does not provide information about the spectral composition or other characteristics of the light. Other measures such as spectral power distribution, colour temperature, and colour rendering index can be used to describe other qualities of light beyond illuminance.
  • The brightness of a light source does not necessarily correspond with its illuminance. For example, a 10-watt light bulb placed next to a person reading a book can provide enough illuminance, while a 1000-watt light bulb located far away may not provide enough light to read by.
  • Illuminance is typically measured in units of lux (lx).
  • Illuminance refers to the amount of light from a natural or artificial light source that falls on a surface. It is usually used to describe the usable light, regardless of the total brightness of the light source.
  • When a book is placed on a table, different levels of illuminance can be observed depending on whether the sky is overcast, the time of day, or whether the surface is indirectly lit.
  • Illuminance is a measure of the amount of light that falls on a surface per unit area. It is determined by the intensity of the light source and the distance from the light source to the surface, but is independent of the characteristics of the surface it strikes, such as its colour or reflectivity.

Illumination

Illumination (lighting) is the deliberate use of light to achieve a practical, aesthetic or physiological effect.

  • Illumination can be provided through artificial light sources such as lamps and light fixtures, or natural illumination by capturing daylight.
  • Daylighting, which involves the use of windows, skylights or light shelves, is sometimes used as the main source of light during daytime in buildings.
  • Specialized forms of artificial lighting have been developed to suit every possible situation and purpose where natural light is not available, such as in underground spaces or during nighttime.
  • The colour temperature of light can affect how colours appear to an observer. Lighting producing a colour temperature below 4000K produces the impression of warmer colours. Lighting producing a colour temperature above 5000K produces the impression of cooler colours.
  • Colour temperature is measured in Kelvin (K) – the lower the number, the warmer the white light appears, while a higher Kelvin will appear cooler. Colour temperature generally ranges from 2700-3300K (warm) to 3300-5300 (cool) – and 6500K is daylight.
  • Illumination (lighting) is the deliberate use of light to achieve a practical, aesthetic or physiological effect.
  • Illumination can be provided through artificial light sources such as lamps and light fixtures, or natural illumination by capturing daylight.
  • Daylighting, which involves the use of windows, skylights or light shelves, is sometimes used as the main source of light during daytime in buildings.
  • Specialized forms of artificial lighting have been developed to suit every possible situation and purpose where natural light is not available, such as in underground spaces or during nighttime.
  • The colour temperature of light can affect how colours appear to an observer. Lighting producing a colour temperature below 4000K produces the impression of warmer colours. Lighting producing a colour temperature above 5000K produces the impression of cooler colours.

Impact parameter

The term impact parameter refers to a scale used on a ray-tracing diagram to measure the point at which incident rays strike the surface of a raindrop. Rays are given a value between 0.0 and 1.0 depending upon their point of impact.

  • For a primary rainbow, all the incident rays of interest strike a raindrop between its horizontal axis (0.0 on the impact parameter scale) and the upper-most point (1.0 on the impact parameter scale). In the second case, the ray grazes the surface at 900 to the normal and continues on its course without deviation.
  • For a secondary rainbow, all the incident rays of interest strike a raindrop between its horizontal axis (0.0 on the impact parameter scale) and the lowest point (1.0 on the impact parameter scale). In the second case, the ray grazes the surface at 900 to the normal and continues on its course without deviation.
  • An impact parameter is useful because it allows the relationship between equidistant incident rays, the angle at which they strike the surface and their angle of refraction to be plotted.

Incandescence

Incandescence is a source of light that occurs naturally as well as artificially. In artificial applications, incandescence is produced by heating a filament in a light bulb until it glows. These incandescent filaments emit radiation across a broad spectrum, including infrared (heat) and some ultraviolet radiation. Only a small portion of this radiation falls within the visible range, which is perceived as light.

  • Incandescent light is produced when electricity flows through a filament, typically made of tungsten, heating it to thousands of degrees Celsius. This intense heat excites the atoms in the filament, causing them to change their energy levels and behaviour:
Energy Levels and Electrons
  • Atoms have distinct energy levels where electrons reside. Imagine these levels like steps on a ladder. The lowest energy level, called the ground state, is like the bottom step. Excitation occurs when atoms absorb energy, such as heat, causing their electrons to jump to higher energy levels (like climbing the ladder).
Energy Absorption and Excitation
  • The heat energy that causes excitation comes in the form of electromagnetic radiation, at visible light or infrared wavelengths. When the radiation collides with atoms, they can transfer their energy to the electrons. If the transferred energy matches the difference between two energy levels in the atom, the electron absorbs it and “excites” to the higher level.
Instability and Return
  • An excited atom is unstable and wants to return to its ground state. It does this by releasing the absorbed energy in different ways:
    • Light Emission: In many cases, the excited atom releases the energy as a photon (light particle) with a specific wavelength corresponding to the energy difference between the initial and final levels. This is how processes like incandescent light, neon signs, and some types of lasers work.
    • Collisions between atoms: The excited atom can transfer its energy to another atom through a collision. This can cause a chain reaction or lead to other physical or chemical reactions.
Colour and Efficiency
  • The colour of incandescent light depends on the filament temperature. Hotter filaments emit more bluish light, while cooler ones glow yellow or orange. However, incandescent light sources are generally less efficient than other lighting technologies, converting a significant portion of their energy into heat rather than light.
Examples
  • Incandescent light: In a hot gas, like the filament in an incandescent bulb, heating excites atoms, causing them to emit visible light, producing their characteristic glow.
  • Fluorescent Light: In fluorescent lamps, UV radiation excites atoms in a gas, the excited atoms then transfer their energy to other atoms, which in turn emit visible light.
  • Aurora: When sunlight excites atoms in the atmosphere, it causes them to emit specific wavelengths of light, resulting in phenomena like the aurora borealis.
Applications and Decline
  • While largely replaced by more efficient options like LED bulbs, incandescent lighting is still used in some applications due to its familiar warm glow and dimming capabilities.
  • However, its use is declining due to its lower energy efficiency and shorter lifespan.

Summary

Incident light

Incident light refers to light that is travelling towards an object or medium.

  • Incident light may come from the Sun, an artificial source or may have already been reflected off another surface, such as a mirror.
  • When incident light strikes a surface or object, it may be absorbed, reflected, refracted, transmitted or undergo any combination of these optical effects.
  • Incident light is typically represented on a ray diagram as a straight line with an arrow to indicate its direction of propagation.
  • A ray diagram is a diagram that uses lines and arrows to represent the path of light, and labels to indicate the angles, directions, and other optical properties of the light.
  • Incident light is an important concept in optics and is used in various fields, including photography, astronomy, and physics.
  • Incident light refers to light that is travelling towards an object or medium.
  • Incident light may come from the Sun, an artificial source or may have already been reflected off another surface, such as a mirror.
  • When incident light strikes a surface or object, it may be absorbed, reflected, refracted, transmitted or undergo any combination of these optical effects.
  • Incident light is typically represented on a ray diagram as a straight line with an arrow to indicate its direction of propagation.

Index of refraction

The refractive index of a medium is a measure of how much the speed of light is reduced when it passes through the medium compared to its speed in a vacuum.

  • The refractive index of a medium is a numerical value and is represented by the symbol n.
  • Because it is a ratio of the speed of light in a vacuum to the speed of light in a medium there is no unit for refractive index.
  • The refractive index of water is 1.333, meaning that light travels at 2/3 the speed in water compared to a vacuum.
  • If the refractive index of a medium is 1.5, for example, light travels at 2/3 the speed through glass compared to a vacuum.
  • As light undergoes refraction, its wavelength changes, but its frequency remains the same.
  • As light undergoes refraction its frequency remains the same.
  • The energy transported by light is not affected by refraction or the refractive index of a medium, but the intensity of the light can be affected.
  • The refractive index of a medium is a measure of how much the speed of light is reduced when it passes through the medium compared to its speed in a vacuum.
  • Refractive index (or, index of refraction) is a measurement of how much the speed of light is reduced when it passes through a medium compared to the speed of light in a vacuum.
  • The concept of refractive index applies to the full electromagnetic spectrum, from gamma-rays to radio waves.
  • Refractive index can vary with the wavelength of the light being refracted. This phenomenon is called dispersion, and it is what causes white light to split into its constituent colours when it passes through a prism.
  • The refractive index of a material can be affected by various factors such as temperature, pressure, and density.

Intensity

Intensity measures the amount of light energy passing through a unit area perpendicular to the direction of light propagation.

  • Intensity measures the amount of energy carried by a light wave or stream of photons.
  • When light is modelled as a wave, intensity is proportional to the square of the amplitude.
  • When light is modelled as a particle, intensity is proportional to the number of photons present at any given point in time.
  • The intensity of light falls off as the inverse square of the distance from a point light source increases.
  • Light intensity at any given distance from a light source is directly related to the power of the light source and the distance from the source.
  • The power of a light source describes the rate at which energy is emitted and is measured in watts.
  • The intensity of light is measured in watts per square meter (W/m²) and is also commonly expressed in lux (lx).
About intensity and brightness
Intensity
  • Intensity refers to the amount of light energy passing through a unit area perpendicular to the direction of light propagation. It measures the concentration of light energy per unit area. Intensity is typically quantified in units such as watts per square meter (W/m²) or lux (lx). In the case of a point source of light, intensity decreases with distance according to the inverse square law.
Brightness
  • Brightness, on the other hand, refers to the subjective perception of how intense or luminous a light source appears to an observer. It is the quality of being bright or emitting or reflecting a lot of light. Brightness is influenced by factors such as the intensity of the light source, the surface area over which the light is distributed, the spectral composition of the light, and the sensitivity of the human eye. Unlike intensity, which is a physical quantity measured objectively, brightness is a perceptual attribute and can vary from person to person.

Interference

Light interference occurs when two or more light waves interact with one another, resulting in a combination of their amplitudes. The resulting wave may increase or decrease in amplitude.

  • A simple form of interference takes place when two plane waves of the same frequency meet at an angle and combine.
  • Light interference is often observed as interference patterns, such as seen in supernumerary rainbows.
  • Interference patterns are produced when the energy of waves combines constructively or destructively. For example, waves on a pond can create interference patterns.
    • Constructive interference occurs when the crest of one wave meets the crest of another wave of the same frequency at the same point. The resulting wave is the sum of the amplitudes of the original waves.
    • Destructive interference occurs when the trough of one wave meets the crest of another wave. The resulting wave is the difference between the amplitudes of the original waves.
  • In the field of optics, a classical wave model based on the Huygens–Fresnel principle is commonly used to explain optical interference.
  • In the field of quantum mechanics, another model called the Path Integral formulation, developed by Richard Feynman, is used to explain light interference.
About Interference patterns
  • Two waves meeting on a pond is a classic example of wave interference. When two waves overlap, their crests and troughs can interact in two ways:
    • Constructive interference: If the crests of both waves line up, they reinforce each other, creating a higher crest in the resulting wave. This leads to a more noticeable wave disturbance at that point.
    • Destructive interference: If one wave’s crest overlaps another’s trough, they partially cancel each other out. The resulting wave will have a smaller amplitude (lower crest) or might appear flat at that specific spot.

A two-point source interference pattern is a pattern that results from the superposition of two waves emanating from two different sources that are in phase with each other. The pattern has the following features:

  • Interference fringes: The interference pattern consists of bright and dark fringes, known as interference fringes. The bright fringes represent constructive interference, where the peaks of the two waves coincide, while the dark fringes represent destructive interference, where the peak of one wave coincides with the trough of the other.
  • Equidistant fringes: The interference fringes are equidistant from each other, with the distance between adjacent bright fringes (or adjacent dark fringes) being the same.
  • Central maximum: The central region of the pattern is the brightest, and corresponds to the point where the two waves are in phase and reinforce each other most strongly.
  • Symmetry: The pattern is symmetric about the midpoint between the two sources.
  • Wavelength dependence: The spacing between the fringes depends on the wavelength of the light, with shorter wavelengths producing fringes that are closer together than longer wavelengths.
  • Amplitude dependence: The spacing between the fringes also depends on the amplitude of the waves, with waves of higher amplitude producing fringes that are further apart than waves of lower amplitude.
  • Angle dependence: The angle between the two sources and the observer’s position relative to the sources can affect the interference pattern, causing it to shift or distort.
  • Light interference occurs when two or more light waves interact with one another, resulting in a combination of their amplitudes. The resulting wave may increase or decrease in amplitude.
  • A simple form of interference takes place when two plane waves of the same frequency meet at an angle and combine.
  • Light interference is often observed as interference patterns, such as seen in supernumerary rainbows.
  • Interference patterns are produced when the energy of waves combines constructively or destructively. For example, waves on a pond can create interference patterns.
    • Constructive interference occurs when the crest of one wave meets the crest of another wave of the same frequency at the same point. The resulting wave is the sum of the amplitudes of the original waves.
    • Destructive interference occurs when the trough of one wave meets the crest of another wave. The resulting wave is the difference between the amplitudes of the original waves.

Interference patterns

About Interference patterns
  • Two waves meeting on a pond is a classic example of wave interference. When two waves overlap, their crests and troughs can interact in two ways:
    • Constructive interference: If the crests of both waves line up, they reinforce each other, creating a higher crest in the resulting wave. This leads to a more noticeable wave disturbance at that point.
    • Destructive interference: If one wave’s crest overlaps another’s trough, they partially cancel each other out. The resulting wave will have a smaller amplitude (lower crest) or might appear flat at that specific spot.

A two-point source interference pattern is a pattern that results from the superposition of two waves emanating from two different sources that are in phase with each other. The pattern has the following features:

  • Interference fringes: The interference pattern consists of bright and dark fringes, known as interference fringes. The bright fringes represent constructive interference, where the peaks of the two waves coincide, while the dark fringes represent destructive interference, where the peak of one wave coincides with the trough of the other.
  • Equidistant fringes: The interference fringes are equidistant from each other, with the distance between adjacent bright fringes (or adjacent dark fringes) being the same.
  • Central maximum: The central region of the pattern is the brightest, and corresponds to the point where the two waves are in phase and reinforce each other most strongly.
  • Symmetry: The pattern is symmetric about the midpoint between the two sources.
  • Wavelength dependence: The spacing between the fringes depends on the wavelength of the light, with shorter wavelengths producing fringes that are closer together than longer wavelengths.
  • Amplitude dependence: The spacing between the fringes also depends on the amplitude of the waves, with waves of higher amplitude producing fringes that are further apart than waves of lower amplitude.
  • Angle dependence: The angle between the two sources and the observer’s position relative to the sources can affect the interference pattern, causing it to shift or distort.

Internal reflection

Internal reflection takes place when light travelling through a denser medium such as water reaches the boundary with a less dense medium such as air and is reflected back into the denser medium.

  • Internal reflection is a common phenomenon with all types of electromagnetic radiation, including visible light.
  • Internal reflection takes place when light reaches the boundary between a medium with a higher refractive index and a medium with a lower refractive index.
  • So, internal reflection takes place when light travels from glass to air at an angle greater than the critical angle, but not when it travels from air to glass.
  • The amount of internal reflection depends upon the angle of incidence as light approaches the boundary. Here are the different outcomes that result from different angles of incidence:
    • At a 00 degree angle of incidence, there is no internal reflection; the light passes straight through the boundary without deviation.
    • As the angle of incidence increases, more and more light is internally reflected and less and less light is refracted at the boundary. This means that less is refracted and so progressively less crosses the boundary into the medium with the lower refractive index.
    • At the critical angle, the light grazes the boundary, and all of it is internally reflected, resulting in no refraction into the second medium.
    • Beyond the critical angle, total internal reflection occurs, and the light is entirely reflected into the first medium.
Example
  • Here is an example. If light travels from water to air where the critical angle is about 48.6 degrees.
    • This means that if light reflects off a fish in a fish tank and then strikes the surface of the water at an angle of less than 48.6 degrees, the angle of incidence determines how much light is internally reflected.
    • If light reflects off a fish in a fish tank and then strikes the water’s surface at an angle of 48.6 degrees or greater, it will experience total internal reflection and no light will pass out of the water and into the air.
  • In reality, light is usually partially refracted and partially reflected because of irregularities in the surface at the boundary. This causes differences in the angle of incidence at different points across the boundary.
  • Internal reflection takes place when light travelling through a denser medium such as water reaches the boundary with a less dense medium such as air and is reflected back into the denser medium.
  • Internal reflection is a common phenomenon with all types of electromagnetic radiation, including visible light.
  • Internal reflection takes place when light reaches the boundary between a medium with a higher refractive index and a medium with a lower refractive index.
  • So, internal reflection takes place when light travels from glass to air at an angle greater than the critical angle, but not when it travels from air to glass.
  • The amount of internal reflection depends upon the angle of incidence as light approaches the boundary. Here are the different outcomes that result from different angles of incidence:
    • At a 00 degree angle of incidence, there is no internal reflection; the light passes straight through the boundary without deviation.
    • As the angle of incidence increases, more and more light is internally reflected and less and less light is refracted at the boundary. This means that less is refracted and so progressively less crosses the boundary into the medium with the lower refractive index.
    • At the critical angle, the light grazes the boundary, and all of it is internally reflected, resulting in no refraction into the second medium.
    • Beyond the critical angle, total internal reflection occurs, and the light is entirely reflected into the first medium.

Internal reflection

Internal reflection takes place when light travelling through a medium such as water fails to cross the boundary into another transparent medium such as air. The light reflects back off the boundary between the two media.

  • Internal reflection is a common phenomenon so far as visible light is concerned but occurs with all types of electromagnetic radiation.
  • For internal refraction to occur, the refractive index of the second medium must be lower than the refractive index of the first medium. So internal reflection takes place when light reaches air from glass or water (at an angle greater than the critical angle), but not when light reaches glass from air.
  • In most everyday situations light is partially refracted and partially reflected at the boundary between water (or glass) and air because of irregularities in the surface.
  • If the angle at which light strikes the boundary between water (or glass) and air is less than a certain critical angle, then the light will be refracted as it crosses the boundary between the two media.
  • When light strikes the boundary between two media precisely at the critical angle, then light is neither refracted or reflected but is instead transmitted along the boundary between the two media.
  • However, if the angle of incidence is greater than the critical angle for all points at which light strikes the boundary then no light will cross the boundary, but will instead undergo total internal reflection.
  • The critical angle is the angle of incidence above which internal reflection occurs. The angle is measured with respect to the normal at the boundary between two media.
  • The angle of refraction is measured between a ray of light and an imaginary line called the normal.
  • In optics, the normal is an imaginary line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
  • If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.

Interneuron

Interneurons are a type of neuron found in the nervous system of animals, including humans, which play a role in processing and communicating information.

  • Interneurons can be classified into different types based on their functions, such as local circuit interneurons and relay interneurons.
    • Local circuit interneurons have short axons and form circuits with nearby neurons to analyse and process information locally.
    • Relay interneurons have long axons and connect circuits of neurons in different regions of the central nervous system, enabling communication and integration of information.
  • Interneurons can be further classified into sub-classes based on their neurotransmitter type, morphology, and connectivity.
  • Interneurons serve as nodes within neural circuits, enabling communication and integration of sensory and motor information between the peripheral nervous system and the central nervous system.
  • The interaction between neurons and interneurons is essential for the proper functioning of the nervous system, enabling the brain to perform complex processes such as perception, cognition, and motor control.
About interneurons and the human eye
  • There are four types of interneurons in the human eye: They are the amacrine cell, bipolar cell, horizontal cell and Müller cell.
  • Interneurons in the human eye form a complex network of interconnections between photoreceptor cells (i.e., rod and cone cells) and retinal ganglion cells.
  • Rod and cone cells are the photoreceptor cells in the human retina that respond to light.
  • Ganglion cells are the retinal neurons that receive and integrate visual information from photoreceptor cells and then transmit it to the brain via the optic nerve.
  • The complex network of interneurons in the human eye plays an important role in the processing and integration of visual information before transmitting it to the brain.
  • This network is also responsible for various visual functions, including spatial filtering, contrast enhancement, and colour opponent processing.
  • Interneurons are a type of neuron found in the nervous system of animals, including humans, which play a role in processing and communicating information.
  • Interneurons can be classified into different types based on their functions, such as local circuit interneurons and relay interneurons.
    • Local circuit interneurons have short axons and form circuits with nearby neurons to analyse and process information locally.
    • Relay interneurons have long axons and connect circuits of neurons in different regions of the central nervous system, enabling communication and integration of information.
  • Interneurons can be further classified into sub-classes based on their neurotransmitter type, morphology, and connectivity.
  • Interneurons serve as nodes within neural circuits, enabling communication and integration of sensory and motor information between the peripheral nervous system and the central nervous system.

Interneurons

About interneurons
  • Interneurons are also referred to as relay neurons, connector neurons, intermediate neurons and local circuit neurons each of which helps to explain their function.
  • Interneurons form nodes within neural circuits, enabling communication between sensory or motor neurons and the central nervous system.
  • Interneurons can be further broken down into two groups: local interneurons and relay interneurons.
    • Local interneurons have short axons and form circuits with nearby neurons to analyse small pieces of information.
    • Relay interneurons have long axons and connect circuits of neurons in one region of the brain with those in other regions.
  • The interaction between interneurons allows the brain to perform complex functions, such as sense-making.

Interneurons & the human eye

About interneurons and the human eye
  • There are four types of interneurons in the human eye: They are the amacrine cell, bipolar cell, horizontal cell and Müller cell.
  • Interneurons in the human eye form a complex network of interconnections between photoreceptor cells (i.e., rod and cone cells) and retinal ganglion cells.
  • Rod and cone cells are the photoreceptor cells in the human retina that respond to light.
  • Ganglion cells are the retinal neurons that receive and integrate visual information from photoreceptor cells and then transmit it to the brain via the optic nerve.
  • The complex network of interneurons in the human eye plays an important role in the processing and integration of visual information before transmitting it to the brain.
  • This network is also responsible for various visual functions, including spatial filtering, contrast enhancement, and colour opponent processing.

Invisible dimensions of rainbows

A typical atmospheric rainbow includes six bands of colour from red to violet but there are other bands of light present that don’t produce the experience of colour for human observers.

  • It is useful to remember that:
  • Radio waves, at the end of the electromagnetic spectrum with the longest wavelengths and the least energy, can penetrate the Earth’s atmosphere and reach the ground but are invisible to human eyes.
  • Microwaves have shorter wavelengths than radio waves, can penetrate the Earth’s atmosphere and reach the ground but are invisible to human eyes.
  • Longer microwaves (waves with similar lengths to radio waves) pass through the Earth’s atmosphere more easily than the shorter wavelengths nearer the visible parts of spectrum.
  • Infra-red is the band closest to visible light but has longer wavelengths. Infra-red radiation can penetrate Earth’s atmosphere but is absorbed by water and carbon dioxide. Infra-red light doesn’t register as a colour to the human eye.
  • The human eye responds more strongly to some bands of visible light between red and violet than others.
  • Ultra-violet light contains shorter wavelengths than visible light, can penetrate Earth’s atmosphere but is absorbed by ozone. Ultra-violet light doesn’t register as a colour to the human eye.
  • Radio, microwaves, infra-red, ultra-violet are all types of non-ionizing radiation, meaning they don’t have enough energy to knock electrons off atoms. Some cause more damage to living cells than others.
  • The Earth’s atmosphere is opaque to both X-rays or gamma-rays from the ionosphere downwards.
  • X-rays and gamma-rays are both forms of ionising radiation. This means that they are able to remove electrons from atoms to create ions. Ionising radiation can damage living cells.
Remember that:
  • All forms of electromagnetic radiation can be thought of in terms of waves and particles.
  • All forms of light from radio waves to gamma-rays can be thought to propagate as streams of photons.
  • The exact spread of colours seen in a rainbow depends on the complex of wavelengths emitted by the light source and which of those reach an observer.