Additive colour

Additive colour is a method of mixing different wavelengths of light to produce other colours.

  • An additive approach to colour is used in the case of emission of light from the screens of mobile phones, computers and televisions.
  • An additive approach to colour is used in the case of the reflection of light off-white, neutral or black surfaces by digital projectors.
  • RGB colour is an additive colour model that combines wavelengths of light corresponding with red, green and blue primary colours to produce other colours.
  • Red, green and blue are called additive primary colours in an RGB colour model because they can be added together to produce all other colours.
  • RGB colour uses three light sources or beams. Each is called a component of the resulting colour,
  • Different colours are produced by varying the intensity of the component colours between fully off and fully on.
  • When fully saturated red, green and blue primary colours are combined they produce white.
  • When any two fully saturated additive primaries are combined they produce a secondary colour: yellow, cyan and magenta.
  • Some RGB colour models can produce over 16 million colours by varying the proportion and intensity of each of the three primary colours.

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.

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.

Chromatic dispersion

Colour

Things appear coloured 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 ability of an organism or machine to distinguish objects based on the wavelengths (or frequencies) 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.

  • Internal reflection is a common phenomenon so far as visible light is concerned but occurs with all types of electromagnetic radiation.
  • Internal reflection takes place when light travelling through a medium strikes the boundary of another medium with a lower refractive index at an angle greater than the critical angle.
  • For example, internal reflection takes place when light reaches air from glass at an angle greater than the critical angle, but not when light reaches glass from air.
  • In general, light will be partially refracted and partially reflected because of irregularities in the surface at the boundary.
  • 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.

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.

  • Dispersion is the result of the relationship between refractive index and wavelength.
  • Every wavelength of light is affected to a different degree by the refractive index of a medium and as a result, changes direction by a different amount eg. when passing from one medium (such as air) to another (such as glass). In the case of white light, the separate wavelengths span out with red at one end and violet at the other.
  • A  familiar example of dispersion is when white light strikes raindrops and a rainbow of colours become visible to an observer.
  • As the light first enters and then exits a droplet it separates into its component wavelengths which the observer perceives as colour.
  • Colour is not a property of electromagnetic radiation, but a feature of visual perception experienced by an 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).

  • An electromagnetic field results from the coupling of an electric and magnetic field.
  • When an electromagnetic field experiences a change in voltage or current its reconfiguration into an electromagnetic wave can be described in terms of wavelength, frequency and energy.
  • An electromagnetic wave can be thought to come into existence when a static electric field experiences a change in voltage or a static magnetic field experiences a change in current producing radiating oscillations of electromagnetic energy that propagate through space.
  • The difference between an electromagnetic field and an electromagnetic wave is that the wave has a non-zero frequency component which is the source of the energy it transports.
  • Electromagnetic radiation is essentially the result of an oscillating electromagnetic field propagating through space.

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