## Angle of incidence

The angle of incidence refers to the angle at which incoming light strikes a surface and is measured between a ray of incoming light and an imaginary line called the normal.

## Angle of reflection

The angle of reflection measures the angle at which light rebounds from a surface after being reflected.

• 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), the boundary between two media.
• The angle of reflection can be used to understand how light will behave when it interacts with different types of surfaces and objects.

## Angle of refraction

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

• The angle of refraction is measured between the bent ray 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.
• Snell’s law is a formula used to describe the relationship between the angle of incidence and the angle of refraction when light crosses the boundary between transparent media, such as water and air or water and glass.

## Anti-solar point

On a sunny day, stand with the Sun on your back and look at the ground, the shadow of your head coincides with the antisolar point.

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

When light separates into its component wavelengths, an observer perceives bands of colour due to the human eye’s sensitivity to different parts of the visible spectrum.

• 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 an atmospheric rainbow contains electromagnetic waves with all possible wavelengths between red and violet, our eyes encounter difficulties in distinguishing between colours within specific regions of this spectrum. For example, all wavelengths between 520 to 570 nanometers may appear to be exactly the same green to most observers.

## Chromatic dispersion

Chromatic dispersion means dispersion according to colour and associated wavelengths of light. Under certain conditions, chromatic dispersion causes light to separate into its component wavelengths producing a rainbow of colours for a human observer.

• Chromatic dispersion is best demonstrated by passing a beam of light through a glass prism.
• A familiar example of chromatic dispersion is when white light strikes raindrops and a rainbow of colours becomes visible to an observer.
• As light first enters and then exits each raindrop, it separates into its component wavelengths which the observer sees as a band of distinct colours.
• Chromatic dispersion can be explained in terms of the relationship between wavelength and refractive index.
• When light propagates from one medium (such as air) to another (such as glass or water) every wavelength of light is affected to a different degree according to the refractive index of the media concerned. As a result, each wavelength changes direction by a different degree. In the case of white light, the separate wavelengths fan out with red on one side and violet on the other.
• Remember that wavelength is a property of electromagnetic radiation, whilst colour is a feature of visual perception.

## Colour

The perception of colour by an observer results from properties of light that are visible to the human eye. The visual experience of colour is associated with terms like red, blue and yellow.

## Colour model

A colour model is the how-to part of colour theory. Together they establish terms and definitions, rules or conventions and a system of notation for encoding colours and their relationships with one another.

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 of objects

A material gets its colour as molecules absorb some wavelengths of light and reflect others. The colour an observer sees corresponds with the reflected wavelengths.

• The part of a molecule that determines the colour an observer sees is called the chromophore.
• The colour produced by a surface or object corresponds with wavelengths of light that are not absorbed during their interaction with the chromophore.

## 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.

• Colour is not a property of electromagnetic radiation, but a feature of visual perception.
• 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.
• 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 photosensitive cone cells in the eye. In essence, different people see the same illuminated object or light source in different ways.
• Colours can be organised and quantified and colour theory helps to make sense of its appearance in different situations.

## Colour wheel

A colour wheel is a circular diagram divided into segments, featuring primary colours, and used to visualize the result of colour mixing.

• Colour wheels can enhance understanding of colour relationships and assist with the accurate selection and reproduction of colours.
• A colour wheel consists of segments representing primary colours. Additional segments are added between them to explore the outcome of mixing adjacent primary colours.
• By adding more segments between existing ones, further mixing of adjacent colours can be explored.
• A colour wheel exploring the additive RGB colour model starts with red, green, and blue primary colours.
• A colour wheel exploring the subtractive CMY colour model starts with cyan, magenta, and yellow primary colours.

## Cone cell

Cone cells, or cones, are a type of neuron (nerve cell) in the retina of the human eye.

• Cone cells are cone-shaped whilst rod cells are rod-shaped.
• Cone cells are responsible for colour vision and function best in bright light, as opposed to rod cells, which work better in dim light.
• 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

Electric and magnetic fields are fundamental forces responsible for generating and transmitting electromagnetic radiation, including visible light.

• All forms of light consist of both electric and magnetic fields oscillating perpendicular to each other and to their direction of propagation.
• Descriptions of light, whether in terms of electromagnetic waves or photons, result from the interaction of electric and magnetic fields.
• When changes in electric and magnetic fields result in electromagnetic waves, they produce synchronized oscillations that travel at right angles to each other at the speed of light  (299,792 kilometres per second).

## Electric field

Dynamic electric fields are a property of photons.  Dynamic electric fields (along with dynamic magnetic fields) are responsible for the transmission of electromagnetic energy, such as visible light.

• Photons are massless particles that carry electromagnetic energy. A photon is a quantum of light.
• The electric fields produced by photons are oscillating, meaning their strength varies between maximum and minimum values over time.
• The frequency of the electric field determines the frequency of the photon. The higher the frequency of the photon, the shorter the wavelength of the photon.

## Electromagnetic field

An electromagnetic field is a more comprehensive entity than its individual electric and magnetic field components.

• Electromagnetic fields are fundamental and intrinsic properties of space.
• Electromagnetic fields are regions of space where the influence of electric and magnetic forces can be detected.
• They play a pivotal role in explaining the transmission of electromagnetic waves (e.g., visible light) across space.
• Changes in a magnetic field result in the emergence of an electric field, and vice versa.
• Electromagnetic fields do not have a specific frequency or wavelength. They are continuous and can exist even without the presence of electromagnetic waves.
• Electric fields and magnetic fields are always perpendicular (at 900) to one another.
• Dynamic electric fields and dynamic magnetic fields are interconnected elements of electromagnetic waves.
• Electromagnetic waves are a specific manifestation of electromagnetic fields in motion.
• Electromagnetic waves can be described as discrete packets of energy called photons, which carry the energy corresponding to the wave’s frequency.
• They are produced by the oscillation of charged particles including electrons, which generates changing electric and magnetic fields that propagate through space.
• Electromagnetic waves are characterized by specific frequencies and wavelengths, which determine the type of electromagnetic radiation (e.g., radio waves, visible light, X-rays) they represent.
• These waves transport energy and information from one location to another.
• Electromagnetic waves travel at the speed of light and exhibit wave-like behaviour, including reflection, refraction, and interference.
• Unlike electromagnetic fields, electromagnetic waves require the presence of both electric and magnetic fields that are in phase with each other.