## Propagation

Wave propagation is any of the ways in which waves travel.

• With respect to the direction of the oscillation relative to the propagation direction, we can distinguish between longitudinal wave and transverse waves. For electromagnetic waves, propagation may occur in a vacuum as well as in a material medium.

https://en.wikipedia.org/wiki/Wave_propagation

## Pure colour

A pure colour is a monochromatic colour.

• Each wavelength of light at full saturation and brightness is perceived as a pure colour.
• A monochromatic colour is a colour produced by a single wavelength of light.
• Colours produced by a narrow band of adjacent wavelengths often appear to be pure colours.
• Spectral colours are the pure colours associated with a natural rainbow.
• Natural rainbow colours include red, orange, yellow, green, blue and violet but the human eye can distinguish many thousands of other pure colours as well as each of these.
• In a continuous spectrum of sufficiently close wavelengths, separate colours are indistinguishable.

## Qualitative

A qualitative measure is a measurement of the quality of something rather than its quantity.

https://en.wikipedia.org/wiki/Observation

## Quantitative

Definition

A quantitative measure is a measurement of the quantity of something rather than its quality.

Explanation

https://en.wikipedia.org/wiki/Observation

Radiant energy is the energy of electromagnetic radiation, the energy carried by light.

• Electromagnetic (EM) radiation can be thought of as a stream of photons, in which case radiant energy can be viewed as photon energy – the energy carried by these photons.
• Alternatively, EM radiation can be viewed as an electromagnetic wave, carrying energy in its oscillating electric and magnetic fields. These two views are completely equivalent and are reconciled to one another in quantum field theory (see wave-particle duality).
• Radiant energy includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays.
• The quantity of radiant energy is measured in terms of radiant flux over time.
• Radiant energy also applies to gravitational radiation. For example, the first gravitational waves ever observed were produced by a black hole collision that emitted about 5.3×1047 joules of gravitational-wave energy.

Radiometry is the science of measurement of radiant energy in terms of absolute power.

• Radiant energy is the electromagnetic energy transported by electromagnetic waves.
• Radiant energy can also be described in terms of elementary particles called photons.
• Radiometric techniques characterize the distribution of the radiation’s power (transfer of energy per unit of time) in space.
• The symbol Qe is often used to denote radiant energy (“e” for “energetic”, to avoid confusion with photometric quantities).
• The SI unit of radiant energy is the joule (J).
• Whilst radiometry deals with electromagnetic radiation, photometry deals with the interaction of light with the human eye.
• Outside of the field of radiometry, electromagnetic energy is referred to using E or W. The term is used particularly when electromagnetic radiation is emitted by a source into the surrounding environment. This radiation may be visible or invisible to the human eye.

## Rainbow

A rainbow is an optical effect produced by illuminated droplets of water. Rainbows are caused by reflection, refraction and dispersion of light in individual droplets and results in the appearance of an arc of spectral colours.

• Rainbows only appear when weather conditions are ideal and an observer is in the right place at the right time.
• Waterfalls, lawn sprinklers and other things that produce water droplets can produce a rainbow.
• A rainbow is formed from millions of individual droplets each of which reflects and refracts a tiny coloured image of the sun towards the observer.
• It is the dispersion of light as refraction takes place that produces the rainbow colours seen by an observer.
• When the sun is behind an observer then the rainbow will appear in front of them.

## Rainbow

rainbow is an optical effect produced by illuminated droplets of water. Rainbows are caused by reflectionrefraction and dispersion of light in individual droplets and results in the appearance of an arc of spectral colours.

• Rainbows only appear when weather conditions are ideal and an observer is in the right place at the right time.
• Waterfalls, lawn sprinklers and other things that produce water droplets can produce a rainbow.
• A rainbow is formed from millions of individual droplets each of which reflects and refracts a tiny coloured image of the sun towards the observer.
• It is the dispersion of light as refraction takes place that produces the rainbow colours seen by an observer.
• If the sun is behind an observer then the rainbow will appear in front of them.
• When a rainbow is produced by sunlight, the angles between the sun, each droplet and the observer determine which ones will form part of a rainbow and which colour each will produce.
• Rainbows always form arcs around a centre point (anti-solar point) because each colour is at a different angle to an observer.
• The axis of a rainbow is an imaginary line drawn between the light source and observer during the period the bow is visible. The anti-solar point is on the same axis.
• If you can see your own shadow and a rainbow at the same time then the anti-solar point, the centre of the rainbow, is aligned with the shadow of your head.
• Seen from the air a rainbow can appear as a complete circle. It is only because the ground around the observer gets in the way that a rainbow produced by sunlight is reduced from a circle to a semi-circle or an arc.
• The sky inside a rainbow is brighter than on the outside because raindrops direct light there too.
• When a single rainbow is observed then red will appear on the outside, followed by orange, yellow, green, blue, with violet on the inside.
• When a double rainbow is observed then the secondary rainbow will be outside the first, it will be less intense, and the colours will be in the reverse order with violet on the outside.

https://en.wikipedia.org/wiki/Rainbow

## Rainbow colours

Rainbow colours are the colours seen in rainbows and in other situations where visible light separates into its component wavelengths and the spectral colours corresponding with each wavelength become visible to the human eye.

• The rainbow colours (ROYGBV) in order of wavelength are red (longest wavelength), orange, yellow, green, blue and violet (shortest wavelength).
•  It is the sensitivity of the human eye to this small part of the electromagnetic spectrum that results in our perception of colour.
• Naming rainbow colours is a matter more closely related to the relationship between perception and language than anything to do with physics or scientific accuracy.
• Even the commonplace colours associated with rainbows defy easy definition. They are concepts we generally agree on, but are not strictly defined by anything in the nature of light itself.
• Whilst the visible spectrum and spectral colour are both determined by wavelength and/or frequency it is our eyes and brains that interpret these differences in electromagnetic radiation that result in our colour perceptions.
• Modern portrayals of rainbows have reduced the number of colours to six – ROYGBV. One reason for this is because it is easier to portray using RGB colour.
• RGB colour is a technology principally used to reproduce colour using digital and electronic equipment. RGB colour is an additive colour model in which red, green and blue light is combined in various proportions to reproduce a wide range of other colours. The name of the model comes from the initials of the three additive primary colours, red, green, and blue.

https://en.wikipedia.org/wiki/ROYGBIV

## Rainbow colours

Rainbow colours are the bands of colour seen in rainbows and in other situations where visible light separates into its component wavelengths and the spectral colours corresponding with each wavelength become visible to the human eye.

• The rainbow colours (ROYGBV) in order of wavelength are red (longest wavelength), orange, yellow, green, blue and violet (shortest wavelength).
• The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to this small part of the electromagnetic spectrum that results in the perception of colour.
• Defining rainbow colours is a question more closely related to the relationship between perception and language than to anything to do with physics or scientific accuracy.
• Even the commonplace colours associated with the rainbow defy easy definition. They are concepts we generally agree on, but are not strictly defined by anything in the nature of light itself.
• Whilst the visible spectrum and spectral colour are both determined by wavelength and frequency it is our eyes and brains that interpret these and create our perceptions after a lot of processing.

https://en.wikipedia.org/wiki/ROYGBIV

## Ray

In a diagram, a light ray is a way of tracing the motion of light, including its direction of travel, and what happens when absorption, dispersion, polarization, reflection, refraction, scattering or transmission take place.

• A light ray is a component of a geometric model (a ray diagram) of the properties of light.
• Whilst the world in general terms literally appear before our eyes, many of the properties of the electromagnetic spectrumare invisible to us.
• Simplified models are used to explore and explain some of the invisible optical phenomena. Ray diagrams represent light as a collection of rays that travel in straight lines and bend when they pass through or reflect from surfaces.
• So a light ray is a diagrammatic representation of a narrow beam of light travelling through a vacuum or medium.
• Light waves are part of a similar model (wave diagram). Wave diagrams imagine light to be a wave, with peaks and troughs and allow other properties to be shown that a ray does not make evident.
• Light can also be described in terms of photons which allows additional properties to be modelled.
• One of the conventions used to represent photons is a  Feynman diagram.

## Ray diagram

A ray diagram (ray tracing) uses a set of drawing conventions and labels to visualise the path that rays of light take in order to understand what happens as they encounter different media, materials or objects.

Ray diagrams are used in geometric optics which treats light as rays that travel in straight lines and change speed and/or direction when they encounter different transparent media.

The aim of a ray diagram is to demonstrate optical phenomena such as absorption, dispersion, polarization, reflection, refractionscattering and transmission.

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

## Reflection

Reflection takes place when incoming light strikes the surface of a medium, obstructing some wavelengths which bounce back into the medium from which they originated.

Reflection takes place when light is neither absorbed by an opaque medium nor transmitted through a transparent medium.

If the reflecting surface is very smooth, the reflected light is called specular or regular reflection.

Specular reflection occurs when light waves reflect off a smooth surface such as a mirror. The arrangement of the waves remains the same and an image of objects that the light has already encountered become visible to an observer.

Diffuse reflection takes place when light reflects off a rough surface. In this case, scattering takes place and waves are reflected randomly in all directions and so no image is produced.

## Reflection

Reflection takes place when incoming light strikes the surface of a medium, obstructing some wavelengths which bounce back into the medium from which they originated.

• Reflection takes place when light is neither absorbed by an opaque medium nor transmitted through a transparent medium.
• When light reflects off a surface, the angle of incidence of an incoming ray as it approaches the surface is equal to the angle of reflection.
• The three laws of reflection are as follows:
• The incident ray, the reflected ray and the normal to a surface all lie on the same plane.
• The angle of the incident ray is equal to the angle which the reflected ray makes with the normal.
• The incident ray and the reflected ray appear on opposite sides of the normal.
• If a reflective surface is very smooth, the reflection is described as being specular or regular.
• Specular reflection occurs when light waves reflect off a smooth surface such as a mirror. The arrangement of the waves remains the same and an image of objects that the light has already encountered become visible to an observer.
• Diffuse reflection takes place when light reflects off a rough surface. In this case, scattering takes place and waves are reflected randomly in many different directions and so no image is produced.
• Reflection can take place regardless of the optical density of the medium through which the incident light is propagating or of the medium it bounces off.

## Refraction

Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels across the interface between one transparent medium and another.

• As light travels from a fast medium such as air to a slow medium such as water it bends toward the normal and slows down.
• As light passes from a slow medium such as diamond to a faster medium such as glass it bends away from the normal and speeds up.
• In a diagram illustrating optical phenomena like refraction or reflection, the normal is a line drawn at right angles to the boundary between two media.
• A fast (optically rare) medium is one that obstructs light less than a slow medium.
• A slow (optically dense) medium is one that obstructs light more than a fast medium.
• The speed at which light travels through a given medium is expressed by its index of refraction.
• If we want to know in which direction light will bend at the boundary between transparent media we need to know:
• Which is the faster, less optically dense (rare) medium with a smaller refractive index?
• Which is the slower, more optically dense medium with the higher refractive index?
• The amount that refraction causes light to change direction, and its path to bend, is dealt with by Snell’s law.
• Snell’s law considers the relationship between the angle of incidence, the angle of refraction and the refractive indices (plural of index) of the media on both sides of the boundary. If three of the four variables are known, then Snell’s law can calculate the fourth.

## Refraction

Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels across the interface between one transparent medium and another.

• As light travels from a fast medium such as air to a slow medium such as water it bends toward the normal and slows down.
• As light passes from a slow medium such as diamond to a faster medium such as glass it bends away from the normal and speeds up.
• In a diagram illustrating optical phenomena like refraction or reflection, the normal is a line drawn at right angles to the boundary between two media.
• A fast (optically rare) medium is one that obstructs light less than a slow medium.
• A slow (optically dense) medium is one that obstructs light more than a fast medium.
• The speed at which light travels through a given medium is expressed by its refractive index (also called (index of refraction).
• If we want to know in which direction light will bend at the boundary between transparent media we need to know:
• Which is the faster, less optically dense (rare) medium with a smaller refractive index?
• Which is the slower, more optically dense medium with the higher refractive index?
• The amount that refraction causes light to change direction, and its path to bend, is dealt with by Snell’s law.
• Snell’s law considers the relationship between the angle of incidence, the angle of refraction and the refractive indices (plural of index) of the media on both sides of the boundary. If three of the four variables are known, then Snell’s law can calculate the fourth.

https://en.wikipedia.org/wiki/Refraction

## Refractive index

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 measure 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.
• If the speed of light in a vacuum = 1. Then the ratio is 1:1.
• The refractive index of water is 1.333, meaning that light travels 1.333 times slower in water than in a vacuum. The ratio is therefore 1:1.333.
• 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.

https://en.wikipedia.org/wiki/Refractive_index

## Refractive index

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.

https://en.wikipedia.org/wiki/Refractive_index

## RGB colour

RGB colour is an additive colour model in which red, green and blue light is combined in various proportions to reproduce a wide range of other colours. The name of the model comes from the initials of the three additive primary colours, red, green, and blue.

• The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. However, because of the way the eye works, we can see many other colours which are produced by mixing colours from different areas of the spectrum. A particularly useful range of colours is produced by mixing red, green and blue light.
• The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
• A spectral colour is a colour corresponding with a single wavelength of visible light, or with a narrow band of adjacent wavelengths.
• Except for the three RGB primary colours, RGB colours are not spectral colours because they are produced by combining colours from different areas of the visible spectrum.
• Magenta is an example of an RGB colour for which there is no equivalent spectral colour.
• RGB colour is a technology used to reproduce colour in ways that are well aligned with human perception. When an observer has separate controls allowing them to adjust the intensity of overlapping red, green and blue RGB primary coloured lights they are able to create a match for an extremely wide range of colours.
• When looking at any modern display device such as a computer screen, mobile phone or video projector we are looking at RGB colour.
• RGB colours are produced:
• On a computer or mobile phone screen:  By Juxtaposing tiny dots of light corresponding with the three primary colours, red, green and blue.
• On a digital projector: By projecting three carefully aligned but separate images, one red, one green and one blue onto a screen. Each image is made up of pixels of different intensity. Where the pixels from each image overlap they produce RGB colour by varying their relative intensity.
• In computer software and apps: By selecting RGB colours using swatches or by selecting RGB colour values.

• An extremely wide range of colours can be produced using RGB colour simply by varying the brightness of the three primary colours by different amounts.
• The RGB colour model itself does not define what is meant by red, green and blue, and so the results of mixing them are relative to the choice of the particular red, green and blue lights that are used. When the exact chromaticity of the red, green, and blue primaries are defined, the colour model then becomes an absolute colour space, such as sRGB or Adobe RGB.
• Look at a screen with large pixels (such as a TV) using a magnifying glass to see the three RGB primary colours. Then step back to see how they produce different colours when all the pixels merge together.
• When working with a computer graphics application such as Adobe Illustrator, tints and shades can be produced by adjusting opacity in conjunction with white and black backgrounds.
• Where an RGB colour model is used to control the output of a display device (computer screen, mobile phone or projector), tints are produced by increased the colour value of each RGB component proportionally, so increasing the intensity of the output, whilst shades are produced by proportionally decreasing the colour value of each RGB component, so decreasing the intensity of the output and dimming the output of the device.
• RGB works by asking three questions of any colour: how red it is (R), how green it is (G), and how blue it is (B).
• The RGB model is popular because it can easily be used to produce a comprehensive palette of 1530 spectral colours.
• This RGB model is particularly useful where the output is to a web page or is to be presented on an RGB display device because of the system of notation that allows exact colours to be specified.

• In the implementation of the RGB colour model used in Adobe Illustrator CC:
• Colours can be selected using swatches which by default are identified by their decimal RGB values (right click a swatch to open Swatch Options).
• Once a swatch has been selected right click on the icon for stoke/fill (top right of Swatches panel) and use the Color Picker to find a colour or adjust RGB (decimal and hex), HSB and CMYK values.

https://en.wikipedia.org/wiki/Comparison_of_color_models_in_computer_graphics

## RGB colour

• To be clear about RGB colour it is useful to remember first that:
• The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
• A spectral colour is a colour corresponding with a single wavelength of visible light, or with a narrow band of adjacent wavelengths.
• The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. However, because of the way the eye works, we can see many other colours which are produced by mixing colours from different areas of the spectrum. A particularly useful range of colours is produced by mixing red, green and blue light.
• RGB colour is an entirely different approach to producing and managing colour.
• RGB colour is an additive colour model in which red, green and blue light is combined in various proportions to reproduce a wide range of other colours. The name of the model comes from the initials of the three additive primary colours, red, green, and blue.
• Except for the three primary colours, RGB colours are not spectral colours because they are produced by combining colours from different areas of the visible spectrum.
• RGB colour provides the basis for a wide range of technologies used to reproduce digital colour.
• RGB colour provides the basis for reproducing colour in ways that are well aligned with human perception.
• When an observer has separate controls allowing them to adjust the intensity of overlapping red, green and blue coloured lights they are able to create a match for a very extensive range of colours.
• When looking at any modern display device such as a computer screen, mobile phone or projector we are looking at RGB colour.
• Magenta is an RGB colour for which there is no equivalent spectral colour.

https://en.wikipedia.org/wiki/Comparison_of_color_models_in_computer_graphics