If there is no light, there is no sight

Use this resource for explanations of the key concepts about the basics of light, how we see and how we perceive the world around us.

Crystalline lens

The internal part of the eye that helps to focus incoming light rays onto the back of the eye. Its shape can be altered by a set of muscles allowing the eye to accommodate for near or far vision.

Depth perception

The ability to see the world around us in 3D. The eyes relay visual cues to the brain, either individually or together, and these are interpreted to give us a 3D perspective.

Light models

Light displays the properties of both waves and particles. Some light properties, such as the photoelectric effect, can only be adequately explained by using the particle model. Others, such as interference, are better explained using the wave model.

Light sources

Light is a form of energy, and there has to be a source from which that energy is released. Incandescent sources produce light as a result of heating or combustion. Luminescent sources produce light by other means.


The rods and cones found in the eye’s retina are sensitive to light. They are the eyes’ photoreceptors. When stimulated, they send impulses to the visual centres of the brain, and in turn, we experience the sensation of sight.

Primary colours

The human eye contains three types of colour receptors known as cone cells. Each colour receptor responds to different ranges of the light spectrum centred on red, green and blue.


The bouncing of light rays off an object. If the reflected rays are parallel to one another, the reflection is called specular. If they are reflected in all directions, the reflection is diffuse.


When passing from one medium into another, light rays undergo a change of speed. For light rays hitting the interface at an angle, this change of speed causes the ray to alter the direction of its path.

Refractive index

The ratio of the speed of light in a vacuum to the speed of light in the medium of interest. For example, soda glass has a refractive index of 1.5. The speed of light in a vacuum is 1.5 times faster than in this type of glass.

Speed of light

The speed of light in a vacuum is 299 792 458 metres per second, and all types of light move at this one speed. In other media, its speed is lower – the denser the medium, the slower the speed.

Vision disorders

Any abnormality of the eye that causes less-than-perfect sight. Common disorders are short-sightedness, long-sightedness, lack of accommodation and astigmatism. Other eye disorders include colour blindness.

    Published 28 March 2012, Updated 14 June 2016