## Additive colour

An additive approach to colour refers to a method of mixing different wavelengths of light to produce other colours.

An additive approach to colour is used to produce the vast array of hues an observer sees on the screens of TV’s, computers and phones.

Colour models that rely on an additive approach to colour include:

## Angle of incidence

The angle of incidence measures the angle at which incoming light strikes a surface.

• The angle of incidence is measured between a ray of incoming light and an imaginary line called the normal.
• In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
• If the boundary between the media is curved, then the normal is drawn at a tangent to the boundary.

## Angle of reflection

The angle of reflection measures the angle at which reflected light bounces off a surface.

• The angle of reflection is measured between a ray of light which has been reflected off a surface and an imaginary line called the normal.
• In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
• If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.

## Angle of refraction

The angle of refraction measures the angle to which light bends as it passes across the boundary between different media.

• The angle of refraction is measured between a ray of light and an imaginary line called the normal.
• In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
• If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.
• Snell’s law is a formula used to describe the relationship between the angles of incidence and refraction when referring to light passing across the boundary between two different transparent media, such as water, glass, or air.
• In optics, the law is used in ray diagrams to compute the angles of incidence or refraction, and in experimental optics to find the refractive index of a medium.

## Anti-solar point

The point on the rainbow axis around which the arcs of a rainbow appear is called the anti-solar point.

• An imaginary straight line can always be drawn that passes through the Sun, the eyes of an observer and the anti-solar point – the geometric centre of a rainbow.
• The idea that a rainbow has a centre corresponds with what an observer sees in real-life.
• Seen in side elevation the centre-point of a rainbow is called the anti-solar point.
• ‘Anti’, because it is opposite the Sun with respect to the location of an observer.
• Unless seen from the air, the anti-solar point is always below the horizon.
• The centre of a secondary rainbow is always on the same axis as the primary bow and shares the same anti-solar point.
• First, second, fifth and sixth-order bows all share the same anti-solar point.

## Bands of colour

An observer perceives bands of colour when visible light separates into its component wavelengths and the human eye distinguishes between some colours better than others.

• The human eye and brain together translate light into colour.
• When sunlight is dispersed by rain and forms a rainbow, an observer often distinguishes red, orange, yellow, green, blue and violet bands of colour.
• Although a rainbow contains electromagnetic waves with all possible wavelengths between red and violet, some ranges of wavelengths appear more intense to a human observer than others.

## Colour

Things appear coloured to an observer because colour corresponds with a property of light that is visible to the human eye. The visual experience of colour is associated with words such as red, blue, yellow, etc.

## Colour model

A colour model is a mathematical system used to describe colours using a set of numeric values.

A colour model is a way of:

• Making sense of the colours we see around us in the world.
• Understanding the relationship of colours to one another.
• Understanding how to mix each type of coloured media to produce predictable results.
• Specifying colours using names, codes, notation, equations etc.
• Organising and using colours for different purposes.
• Using colours in predictable and repeatable ways.
• Working out systems and rules for mixing and using different types of colour.
• Creating colour palettes, gamuts and colour guides.

## Colour vision

Colour vision is the human ability to distinguish between objects based on the wavelengths of the light they emit, reflect or transmit. The human eye and brain together translate light into colour.

• The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. Light, however, is rarely of a single wavelength, so an observer will usually be exposed to a range of different wavelengths of light or a mixture of wavelengths from different areas of the spectrum.
• Colours can be measured and quantified in various ways; indeed, a person’s perception of colour is a subjective process whereby the brain responds to the stimuli that are produced when incoming light reacts with several types of cone cells in the eye. In essence, different people see the same illuminated object or light source in different ways.

## Colour wheel

A colour wheel is a diagram based on a circle divided into segments. The minimum number of segments is three with a primary colour in each. Segments added between the primaries can then be used to explore the result of mixing adjacent pairs of primary colours together. Additional segments can then be added between all the existing segments to explore the result of mixing further pairs of adjacent colours.

• 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 rainbow colours.
• Colour wheels are often used in technologies which reproduce colour in ways that match the light sensitivity of the three different types of cone cells and the rod cells in the human eye.
• Colour wheels exploring additive colour models and subtractive colour models use different sets of primary colours.
• An RGB colour wheel, used to explore additive mixing of light, starts with red, green and blue primary colours.
• The colours produced in between the primary colours in a colour wheel are called secondary colours.
• The colours produced in between the secondary colours in a colour wheel are called tertiary colours.
• A CMY colour wheel, used to explore subtractive mixing of pigments and inks (used in digital printing) starts with cyan, magenta and yellow primary colours.
• An RYB colour wheel used to explore the subtractive mixing of art pigments and paints starts with red, yellow and blue primaries.
• The colour wheels described above all depend on trichromatic colour vision which involves three receptor types (cone cells) processing colour stimuli.

## Cone cell

Cone cells, or cones, are one of three types of photoreceptor cells (neurons) in the retina of the human eye. They are responsible for colour vision and function best in relatively bright light, as opposed to rod cells, which work better in dim light.

• Cone cells are cone-shaped whilst rod cells are rod-shaped.
• Cone cells are most concentrated towards the macula and densely packed in the fovea centralis, but reduce in number towards the periphery of the retina.
• There are believed to be around six million cone cells in the human retina.

## Critical angle

The critical angle for light approaching the boundary between two different media is the angle of incidence above which it undergoes total internal reflection. The critical angle is measured with respect to the normal at the boundary between two media.

## Dispersion

Dispersion (or chromatic dispersion) refers to the way that light, under certain conditions, separates into its component wavelengths and the colours corresponding with each wavelength become visible to a human observer.

## Electric and magnetic fields

An electric field is created by a change in voltage (charge). The higher the voltage the stronger the field.

A magnetic field is created when electric current flows. The greater the current the stronger the magnetic field.

• A change in an electric field induces a change in the magnetic field.
• A change in a magnetic field induces a change in the electric field.
• An electromagnetic wave is the result of the interaction of electric and magnetic fields.
• An electromagnetic wave can be propagated when either the charge of an electric field changes or when the current of a magnetic field changes.
• Once an electromagnetic wave propagates outward it cannot be deflected by an external electric or magnetic field.

## Electric field

An electric field is created by a change in voltage (charge). The higher the voltage the stronger the field.

• Whilst an electric field is created by a change in voltage (charge), a magnetic field is created when electric current flows. The greater the current the stronger the magnetic field.
• An electromagnetic wave is the result of the interaction of an electric and magnetic field because an electric field induces a magnetic field and a magnetic field induces an electric field.
• An electromagnetic wave can be induced when either the charge of an electric field changes or when the current of a magnetic field changes or when they both change together.
• The waveform, wavelength and frequency of an electromagnetic wave result from the rapid periodic succession of transitions between the electrical and magnetic components and the forward propagation of the wave through space.
• When electric and magnetic fields come into contact to form electromagnetic waves they oscillate at right angles to one another.
• The direction of propagation of an electromagnetic wave is at right angles to the electric and magnetic fields.
• The velocity at which electromagnetic waves propagate in a vacuum is the speed of light which is 300,000 metres per second.
• Once an electromagnetic wave propagates outward it cannot be deflected by an external electric or magnetic field.
• The reason an electromagnetic wave does not need a medium to propagate through is that the only thing that is waving/oscillating is the value of the electric and magnetic fields.

## Electromagnetic field

An electromagnetic field can be thought of as a single more complete object than its component electric and magnetic field. It propagates through space in the form of bundles of energy called photons which are configured as electromagnetic waves, the force carriers of radiant energy (electromagnetic radiation).

https://en.wikipedia.org/wiki/Electromagnetic_radiation>

## Electromagnetic radiation

Electromagnetic radiation is a type of energy more commonly simply called light. Detached from its source, it is transported by electromagnetic waves (or their quanta, photons) and propagates through space at the speed of light.

• Electromagnetic radiation (EM radiation or EMR) includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays.
• Man-made technologies that produce electromagnetic radiation include radio and TV transmitters, radar, MRI scanners, microwave ovens, computer screens, mobile phones, all types of lights and lamps, electric blankets, electric bar heaters, lasers and x-ray machines.
• At the quantum scale of electromagnetism, electromagnetic radiation is described in terms of photons rather than waves. Photons are elementary particles responsible for all electromagnetic phenomena.
• The term quantum refers to the smallest quantity into which something can be divided. A quantum of a thing is indivisible into smaller units so they have no sub-structure.  A photon is a quantum of electromagnetic radiation.
• A single photon with a wavelength corresponding with gamma rays might carry 100,000 times the energy of a single photon of visible light.