Glossary

Definitions of technical words and phrases used in this site. Clicking on some gives detailed explanations. Underlined links are to other definitions on this page.



Aberration: The change in angle of light rays between observers moving with different velocities.

c: In physics, lower case "c" is used to represent the speed of light, such that c = 299 792 458 meters per second. A certain fraction of the speed of light can be represented by a fraction multiplying c: e.g. half the speed of light is 0.5c.

Distortion: The change in apparent shape or position of an object due to different parts of it being seen at different times.

Doppler effect: The change in frequency of light rays between observers moving with different velocities. Named after the Austrian physicist Christian Doppler.

Frequency: The number of complete cycles a wave, such as light, goes through in a second.

Infrared light: Light beyond the visible range. The color of light is determined by the frequency of its waves. Infrared light has lower frequency than visible light.

Headlight effect: The change in intensity, or brightness, of light rays between observers moving with different velocities.

Length contraction: The relativistic effect whereby lengths of moving objects, along their direction of motion, are reduced.

Observer: Someone or something making measurements, e.g. a camera or an eye.

Photon: A particle, or energy quantum, of light.

Postulate: Something which is assumed to be true for the purposes of deducing consequences. Whether it is actually true or not may be determined from the truth of these consequences.

Relativity of simultaneity: In relativity, the simultaneity of spatially separated events depends on the observers relative motion.

Simultaneity: Happening at the same time.

Time dilation: The relativistic effect whereby moving clocks runs slow.

Ultraviolet light: Light beyond the visible range. The color of light is determined by the frequency of its waves. Ultraviolet light has higher frequency than visible light.

Velocity: The speed and direction of motion.

View-sphere: The view all around us. Compare with the camera or eye view which sees only part of the view-sphere.