Chromatic dispersion

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.

Dictionary of light, colour & vision

This DICTIONARY OF LIGHT, COLOUR & VISION contains a vocabulary of closely interrelated terms that underpin all the resources you will find here at

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Diffuse reflection

Most objects produce diffuse reflections as light scatters off their surfaces in random directions. It is often almost impossible to pick out the shape or colour of objects in a diffuse refection.

About diffuse reflection
About specular reflection
  • Specular reflection, or regular reflection, is the mirror-like reflection of light from a surface in which everything appears as clearly as when viewed directly.
  • Specular reflection occurs when the irregularities on the surface of an object are smaller than the wavelengths of the incident light.


Diffusion refers to spreading something more widely.  When light undergoes diffusion it becomes less concentrated and spreads out.

About diffusion
  • Diffusion results from any material that scatters light during reflection or transmission.
  • Scattering takes place when streams of photons (or waves of light) are deflected in different directions.
  • All objects obey the law of reflection on a microscopic level. If the irregularities on the surface of an object are larger than the wavelength of light, the light undergoes diffusion.
  • Transparent and translucent materials transmit diffuse light unless their surfaces are perfectly flat and their interiors are free of any foreign material.
  • When diffuse light falls on an object or scene it produces softened effects without sharp detail.
  • A mirror-like reflection free of the effects of diffusion is called a specular reflection.

Digital printing

Digital printing usually involves mapping the colours in a digital file (JPEG, PNG, SVG) to the CMYK colour model and printing the data onto paper using cyan, magenta, yellow and black inks.

About digital printing
  • Digital printers typically overlay highly reflective white paper with cyan, magenta, yellow and black inks or toner.
  • Printing has a smaller gamut than TV, computer and phone screens which rely on light emission, rather than reflection.
  • Digital displays produce comparatively brighter colours than printers because the amplitude of each wavelength of light is larger than can be achieved by a printer.
  • Digital printers produce dull and less intense colours than displays because the amplitude of each wavelength of light is smaller when light is reflected off paper through inks.
  • A display device, such as a computer screen, starts off dark and emits red, green and blue light to produce colour.
  • CMYK inks are the standard for colour printing because they have a larger gamut than RGB inks.
  • Highlights are produced on digital printers by printing without black to allow the maximum amount of light possible to shine through and reflect off the paper.
  • Mid tones rely on the brightness and transparency of the inks and the reflectivity of the paper to produce fully saturated colours.
  • Shadows are produced by adding black to both saturated and desaturated hues.


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.

About dispersion


In the field of optics, dispersion is shorthand for chromatic dispersion and 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.

Display device

RGB digital display devices are a feature of TV’s, computers, phones, monitors and projectors.

About digital displays
  • A digital display is an output device for the presentation of information in a visual form. When the input information is in the form of a digital electrical signal, the display is called an electronic display.
  • An RGB digital display adjusts the colour values of the red, green and blue pixels within each addressable component of the screen to produce an image.
  • Fully saturated hues (colours) rely on the intensity (brightness) of each group of pixels.
  • Tints of any hue are produced by increasing the colour value sent to each RGB component in step with one another, which increases the brightness of the output without favouring a particular hue.
  • Shades are produced by decreasing the value sent to each RGB component so dimming the device without affecting the colour of images on the screen.

Distance to, size and duration of rainbows

Distance to a rainbow

Rainbows are formed from the millions of individual raindrops that happen to be in exactly the right place at the right time, so it is difficult to be precise about how far away a rainbow is.

  • Because a rainbow is a trick of the light rather than a solid material object set in the landscape it has no fixed position and is at no fixed distance from an observer. Instead, rainbows move as the Sun and the observer move or as curtains of rain cross the landscape.
  • Because a rainbow is composed of light reflecting off and refracting in millions of individual raindrops it might be fair to say that the distance to a rainbow is the distance to the location of the greatest concentration of raindrops diverting photons towards an observer.
  • An observer cannot easily estimate the distance to a raindrop or a curtain of rain along their line of sight but the position of clouds or objects in the landscape can help to determine where rain is falling.
  • The position of a rainbow is primarily determined by angles. The angles are constants and result from the physical properties of light and water droplets, not least the laws of reflection and refraction.
  • As an observer moves, the rainbow they see moves with them and the angles are preserved.
Size of a rainbow
  • Just as the visual impression of the size of the moon depends on how near it is to the horizon, the apparent diameter of a rainbow is also affected by other features in the landscape.
Duration of a rainbow
  • A rainbow may be visible for minutes on end before receding slowly into the distance. In other situations, a rainbow may appear one moment and be gone the next.

Electromagnetic waves

The form and composition of rainbows are often explained in terms of electromagnetic waves.


Electromagnetic waves consist of coupled oscillating electric and magnetic fields orientated at 900 to one another. (Credit:

  • Electromagnetic waves can be imagined as oscillating electric (E) and magnetic (B) fields arranged at right angles to each other.
  • In the diagram above, the coupled electric and magnetic fields follow the y-axis and z-axis and propagate along the x-axis.
  • This arrangement is known as a transverse wave which means the oscillations are perpendicular to the direction of travel.
  • By convention, the electric field is shown in diagrams aligned with the vertical plane and the magnetic field with the horizontal plane.
  • In normal atmospheric conditions the geometric orientation of the coupled y-axis and z axis is random, so the coupled fields EB may be rotated to any angle.