Illuminance

The term Illuminance refers to the quantity of light that falls on a physical surface. Illuminance is typically used to define the usable light supplied by a natural or artificial light source regardless of its total luminosity.

  • If we place a book on a table, then different levels of illuminance can be observed when the sky is overcast, under the mid-day sun, in moonlight or artificial light.
  • Illuminance is independent of the surface that light falls upon. It is a description of the qualities of the light itself.
  • Illuminance is something that can be measured and so is an objective term.
  • The implication of this is that a 10-watt light-bulb is a fairly powerful source of light if it is placed next to an observer reading a book. But even a 1000-watt light-bulb will not provide enough light to read by if it is located a hundred metres away.

Summary

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Illumination

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

  • Illumination might be provided through the use of artificial light sources like lamps and light fixtures, or natural illumination by capturing daylight.
  • Daylighting (using windows, skylights, or light shelves) is sometimes used as the main source of light during daytime in buildings.
  • Specialised forms of artificial lighting have been developed to suit every possible situation and purpose where natural light is not available.

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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.

Incident light

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

  • Incident light may have travelled from the Sun or an artificial source or may have already been reflected off another surface such as a mirror.
  • When incident light strikes a surface or object in may be absorbed, reflected, refracted, transmitted or undergo any combination of these optical affects.
  • Incident light is often shown on a ray diagram as a single line with an arrow to indicate its direction of propagation.
  • A ray diagram is a drawing that uses conventions and labels to visualise the behaviour of light in different situations.

Summary

Index of refraction

The refractive index of a medium is the amount by which the speed (and wavelength) of electromagnetic radiation (light) is reduced compared with the speed of light in a vacuum.

  • Refractive index (or, index of refraction) is a measurement of how much slower light travels through any given medium than through a vacuum.
  • The concept of refractive index applies to the full electromagnetic spectrum, from gamma-rays to radio waves.
  • 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. The ratio is therefore 1:1.333.
  • If 1 is divided by 1.333 we find that light travels at 0.75 the speed through glass compared to a vacuum.
  • As light undergoes refraction its wavelength changes as its speed changes.
  • 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.
  • The colour of refracted light perceived by a human observer does not change during refraction because the frequency of light and the amount of energy transported remain the same.

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Intensity

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 quantity of photons present at any given point in time.
  • The intensity of light 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, where the area is measured on a plane perpendicular to the direction of propagation of the 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).

Interference

Light interference occurs when two or more light waves interfere with one another causing the combined amplitudes of the waves to either increase or decrease.

  • A simple form of interference takes place when two plane waves of the same frequency intersect at an angle.
  • Light interference is often evident in the appearance of interference patterns such as those that produce supernumerary rainbows.

 

In general terms, interference patterns are produced by a process of energy redistribution. In the case of waves on a pond, the energy gained through constructive interference is lost as a result of destructive interference.

  • Constructive interference occurs when the crest of one wave meets the crest of another wave of the same frequency at the same point. The outcome is that the amplitude of the resulting wave is the sum of the amplitudes of the original waves.
  • Destructive interference occurs when the trough of one wave meets the trough of another wave. The outcome is that the amplitude of the resulting wave is equal to the difference in the amplitudes of the original waves.

Interference of light is a unique phenomenon in that we can never observe superposition of the EM field directly. Superposition in the EM field is an assumed and necessary requirement, fundamentally 2 light beam pass through each other and continue on their respective paths. Light can be explained classically by the superposition of waves, however a deeper understanding of light interference requires knowledge of wave-particle duality of light which is due to quantum mechanics. Prime examples of light interference are the famous double-slit experiment, laser speckle, anti-reflective coatings and interferometers. Traditionally the classical wave model is taught as a basis for understanding optical interference, based on the Huygens–Fresnel principle however an explanation based on the Feynman path integral exists which takes into account quantum mechanical considerations.

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.

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.

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.

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Interneuron

Interneurons are a form of neuron in the human central nervous system and are responsible for the processing and communication of information.

  • Interneurons can be classified as local circuit neurons and relay neurons:
    • Local circuit 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 central nervous system with those in other regions.
  • Interneurons form nodes within neural circuits, enabling communication between sensory or motor neurons and the central nervous system.
  • The interaction between neurons and interneurons are essential to our living bodies but also enable the brain to perform highly complex processes such as sense-making.

Summary

About interneurons and the human eye

Interneurons

Rod and cone photoreceptors within the retina of the human eye encode light into electrical signals that are transmitted via a complex network of interneurons to ganglion cells, which then forward visual information via the optic nerve to the brain.

About interneurons
  • Interneurons form nodes within the neural circuits, enabling communication between sensory or motor neurons within the central nervous system.
  • Visual processing in the retina of the human eye depends on coordinated signalling by interneurons.
  • Interneurons are sometimes referred to as local interneurons and relay interneuron.
    • 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.
References
  • https://en.wikipedia.org/wiki/Interneuron
  • https://en.wikipedia.org/wiki/Central_nervous_system

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:
    • Each band of wavelengths within the electromagnetism spectrum (taken as a whole) is composed of photons that produce different kinds of light.
    • Remember that light can be used to mean visible light but can also be used to refer to other areas of the electromagnetism spectrum invisible to the human eye.
    • Each band of wavelengths represents a different form of radiant energy with distinct properties.
    • The idea of bands of wavelengths is adopted for convenience sake and is a widely understood convention. The entire electromagnetic spectrum is, in practice, composed of a smooth and continuous range of wavelengths (frequencies, energies).
  • 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.