Radiant energy

  • 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.

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

Radiometry

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.

  • Atmospheric 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 band of 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 the rainbow they see and which colour each droplet will produce.
  • Rainbows always form arcs around a single centre point (anti-solar point) with each colour at a slightly different angle to an observer.
  • The axis of a rainbow is an imaginary line drawn between the light source and the anti-solar point of a rainbow with the observer in between.
  • If you can see your own shadow and a rainbow at the same time then the rainbow always has the shadow of your head as its centre.
  • 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 scatter diffuse light of every wavelength inwards towards the centre but none is directed outwards.
  • When an observer sees a single rainbow, red appears on the outside, followed by orange, yellow, green, and blue, with violet on the inside.
  • When an observer sees a double rainbow, the secondary rainbow is outside the first and forms a wider, paler band of colours with violet on the inside.

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, and what happens when it encounters different media.

  • The field of geometric optics uses the idea that light is made up of rays when describing and explaining what happens when it encounters different media.
  • Light rays are not real, they are a concept used to produce an idealised explanation of light.
  • Ray diagrams use straight lines and arrows to show how light propagates.
  • In the natural world light is not really made up of rays. More accurate descriptions of the properties of light use terms such as photons or waves.
  • So a light ray is a diagrammatic representation of a narrow beam of light travelling through a vacuum or medium.
  • The nearest thing to a light ray in the real world is a narrow focused beam of light produced by a laser.

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)

Real-life raindrops

In real-life, full-size raindrops don’t form perfect spheres because they are composed of water which is fluid and are only held together by surface tension.

  • In normal atmospheric conditions, the air a raindrop moves through is itself in constant motion and even at a cubic metre scale or smaller, it is composed of areas at different airflows, temperatures and pressure.
  • As a result of turbulence, a raindrop is rarely in free-fall because it is buffeted by the air around it, accelerating or slowing as conditions change from moment to moment.
  • Raindrops start to form high in the atmosphere around tiny particles called condensation nuclei — these can be composed of little pieces of salt left over after seawater evaporates, or particles of dust or smoke.
  • Raindrops form around condensation nuclei as water vapour cools producing clouds of tiny droplets that start off roughly spherical.
  • Surface tension is the tendency of liquids to shrink to the minimum possible surface area.
  • At water-air interfaces, the surface tension that holds water molecules together results from them being attracted to one another more than to the nitrogen, oxygen, argon or carbon dioxide molecules that make up our atmosphere.
  • As clouds of water droplets begin to form, they are between 0.0001 and 0.005 centimetres in diameter.
  • As soon as droplets form they start to encounter more vapour and collide with one another. As larger droplets bump into other smaller droplets they increase in size — this is called coalescence.
  • Once they are big and heavy enough they begin to fall and continue to grow. Droplets can be thought to be raindrops once they reach 0.5mm in diameter.
  • Sometimes, gusts of wind (updraught) force raindrops back into the clouds and coalescence starts over.
  • As full-size raindrops fall they lose some of their rounded shape. The bottom becomes flattened due to wind resistance whilst the top remains rounded.
  • Large raindrops are the least stable, so once a raindrop is over 4 millimetres it may break apart to form smaller more regularly shaped drops.
  • In general terms, raindrops are different sizes for two primary reasons,  initial differences in particle (condensation nuclei) size and different rates of coalescence.
  • As raindrops near the ground, the biggest are the ones that bumped into and coalesced with the most droplets.

Reflectance

The reflectance of the surface of a material is its effectiveness in reflecting radiant energy.

  • Reflectance is the fraction of incident electromagnetic power that is reflected at the boundary. Power = energy x time.
  • Reflectance is a component of the response of a material to the electromagnetic properties of light, so a function of its:
  • Given that reflectance is a directional property, most surfaces can be divided into those that give specular reflection and those that give diffuse reflection.
  • For specular surfaces, such as glass or polished metal, reflectance is nearly zero at all angles except at the angle visible to an observer.
  • For diffuse surfaces, such as matte white paint, reflectance is uniform in all directions so radiation is reflected at all angles equally or near-equally.
  • Most practical objects exhibit a combination of diffuse and specular reflective properties.

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.

Reflections off raindrops

Not all incident light striking a raindrop crosses the boundary into the watery interior of a droplet. Some of the incident light is reflected off the surface and a small proportion of that travels towards the observer.

  • Incident light reflected off the surface facing an observer undergoes neither refraction nor dispersion.
  • Because the outside surface of a raindrop forms a shiny convex mirror, reflected light diverges in every possible direction depending on its initial point of impact.
  • Just as raindrops form the coloured arc of a primary rainbow, they can also reflect white light towards an observer.
  • White light reflected towards an observer off the outside of raindrops helps to account for why the sky on the inside of a rainbow (between its centre and coloured arcs) appears brighter and lighter than the area of sky outside.

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 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 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.

 

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

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

RGB colour model

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 human eye and light
Red, green and blue light
  • Because of the way the eye works, we can see all the colours of the visible spectrum by mixing red, green and blue lights at different intensities.
  • Red, green and blue are the three primary colours of the RGB colour model.
  • The RGB colour model replicates the response of light-sensitive cone cells in the retina at the back of our eyes sense colour.
  • Mixing wavelengths of light corresponding with the RGB primaries fools the eye into seeing almost any imaginable colour.
Trichromatic colour vision (Trichromacy)

The trichromatic colour theory explains the system the human eye uses to see colour.

  • Trichromatic colour theory is based on the presence of three types of light-sensitive cone cells in the retina at the back of our eyes, each sensitive to a different spread of colour.
  • All the colours we observe result from the simultaneous response of all three types of cones.
  • The sensitivity of cone cells is the physiological basis for trichromatic colour vision in humans.
  • The fact that we see colour is, in the first instance, the result of interactions among the three types of cones, each of which responds with a bias towards its favoured wavelength within the visible spectrum.
  • The result is that the L, M and S cone types respond best to light with long wavelengths (biased towards 560 nm), medium wavelengths (biased towards 530 nm), and short wavelengths (biased towards 420 nm) respectively
RGB and digital devices
  • RGB colour is deeply embedded in many contemporary technologies.
  • 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.
  • 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.
RGB colour model in practice
  • RGB colour model works in practice 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 vivid hues simply by adjusting the intensity of the three primaries.
  • When the saturation or brightness of a hue needs to be adjusted it is sometimes easier to switch to the HSB colour model.
RGB colour values
  • RGB colour values are expressed as decimal triplets (yellow = 255, 255, 0) or hexadecimal triplets (green = #00FF00 ) and are used in software applications to select specific colours.
  • In both cases, the triplets determine the amount of red, green and then blue used to produce a specific colour.
  • A decimal triplet is made up of three numbers between 0 and 255 divided by commas. A hexadecimal triplet starts with a # sign followed by three two-digit numbers with values between  00 and FF written without spaces between.
References
  • https://en.wikipedia.org/wiki/Comparison_of_color_models_in_computer_graphics