Diffraction of electromagnetic radiation refers to various phenomena that occur when a light wave encounters an obstacle or opening.

  • Diffraction and interference are phenomena associated with all kinds of waves. Electromagnetic waves are a special case however because of their unique behaviour.
  • Diffraction deals with the way light bends around the edges of obstacles into regions that would otherwise be in shadow.
  • Interference deals with the way that light behaves during the diffraction process.
  • Diffraction can be produced by the edges or by a hole (aperture) in any opaque surface or object.
  • Diffraction causes a propagating wave to produce a distinctive pattern as light waves interfere with one another. The resulting pattern becomes visible if diffracted light subsequently strikes a surface.
  • Diffraction produces a circular pattern of concentric bands when a narrow beam of light passes through a small circular aperture and then strikes a flat surface.
  • In classical physics, an explanation of the diffraction of electromagnetic waves treats each point at which a propagating wavefront encounters the edge of an obstacle as a site at which a new spherical wavelet supersedes the original waveform.
  • Separate spherical wavelets bend independently of one other beyond the site at which an obstacle is encountered. However, interference between them alters the way they bend and the distance they must travel before striking a surface.
  • Explanations that describe the process of diffraction and interference patterns belong to Wave Theory and are the result of more than two centuries of study in the field of optics.
  • In modern quantum mechanics, diffusion is explained by referring to the wave function and probability distribution of each photon of light as it encounters the corner of an obstacle or the edge of an aperture.
  • Wave functions and probability distributions are part of mathematical formulations of the outcomes of all possible measurements of a photon’s behaviour in the course of diffraction.


Radiation is energy that comes from a source and travels through space at the speed of light.

  • Radiant energy has an electric field, and a magnetic field and may be described in terms of electromagnetic waves or in terms of bundles of photons travelling in a wave-like pattern.
  • Visible light is a form of radiation often described in terms of either electromagnetic waves or photons.
  • Types of radiation with the highest energy include ultraviolet radiation, x-rays, and gamma rays.
  • When x-rays or gamma-rays interact with atoms, they can remove electrons which destabilises them and make them radioactive.
  • Radioactivity is the spontaneous release of energy from an unstable atom as it returns to a stable state.
  • Ionizing Radiation is the energy that comes out of a radioactive atom.

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 lightcolourvision.org.

  • Each term has its own page in the DICTIONARY and starts with a DEFINITION.
  • Bullet points follow that provide both context and detail.
  • Links embedded in the text throughout the site (highlighted in blue) take you directly to DICTIONARY entries.
  • Shorter SUMMARIES of terms appear on DIAGRAM PAGES under the heading SOME KEY TERMS. These pop-up entries strip definitions back to basics and can be viewed without leaving the page.


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

  • Diffusion results from any medium or material that scatters light during refraction, 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.

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.


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.

Digital screen

A digital screen (or digital display) is an output device for the presentation of information in a visual form.

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

  • Digital screens use RGB-related colour models to display visual information.
  • The colour spaces, and so the range of colours, different screens can display depends on their technology and specifications.
  • RGB digital screens adjust the intensity of red, green and blue light-emitting diodes (LEDs) within each addressable component of the screen to produce pixels of colour that together produce an image.
  • Fully saturated hues (colours) are produced when pixels in an area of the screen are at maximum intensity (brightness).
  • Darker tones of any hue are produced by decreasing the intensity of light produced by each pixel so producing a dimmed version of a colour.
  • Black is produced by completely turning off the LEDs within a pixel.

Digital printing

Digital printing uses the CMYK colour model along with cyan, magenta, yellow and black inks to output digital files onto paper and other sheet materials.

All modern printers have built-in colour profiles that enable both device-dependent and device-independent RGB colour spaces to be converted to CMYK.

  • Digital printers typically overlay highly reflective white paper with cyan, magenta, yellow and black inks or toner.
  • CMYK is a subtractive colour model so is suited to working with inks.
  • Printing has a smaller gamut than TV, computer and phone screens which rely on light emission, rather than reflection of light off sheets of paper.
  • 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.
  • https://en.wikipedia.org/wiki/Digital_printing

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 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.


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.

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. The result is that each wavelength changes direction and speed by a different amount.
  • 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 becomes 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.