Physics Glossary – R


Radar is an acronym for radio detection and ranging. It is a system used to detect, range (determine the distance) and map objects such as aircraft and rain. Strong radio waves are transmitted, and a receiver listens for reflected echoes. By analyzing the reflected signal, the reflector can be located, and sometimes identified. Although the amount of signal returned is tiny, radio signals can easily be detected and amplified.


The radian measure of the angle is the length of the arc cut out by the angle, divided by the circle’s radius. Therefore 2π radians is equal to 360° (a full circle), so one radian is about 57.296° and one degree is π/180 radians.


Is the emission of radiation from an unstable atomic nucleus. This emission of energy is called radioactive decay. The radiation can be emitted in the form of a positively charged alpha particle (a), a negatively charged beta particle(ß), or gamma rays .

Radio Telescope

In contrast to an ordinary telescope, which produces visible light images, a radio telescope “sees” radio waves emitted by radio sources located anywhere in the Universe, typically by means of a large parabolic (“dish”) antenna, or arrays of them. The best-known (and largest) radio telescope is in Arecibo, Puerto Rico. A well-known radio telescope being an array of antennae is the Very Large Array (VLA) in Socorro, New Mexico. One of the oldest and most famous is Jodrell Bank Observatory, near Mancester in the UK. The largest (100-meter diameter) and most famous radio telescope in Europe is in Effelsberg, Germany.

Ram Jet

When air enters a jet engine its speed decreases and its pressure increases, called ram compression effect. At high speeds this process can be fairly effective, and can provide enough compression to run an engine all on its own. Typically the speed needed to make this process work effectively is above 600mph, and doesn’t beat traditional designs until supersonic (faster than the speed of sound).

Raman Effect

The scattering of monochromatic light as it passes through a transparent material. This effect is very small about one out of a million photons of light are scattered. Discovered by Chandrasekhara Venkata Raman in 1928.


Refraction is the change in direction of a wave due to a change in velocity. It happens when waves travel from a medium with a given refractive index to a medium with another. At the boundary between the media the wave changes direction, its wavelength increases or decreases but frequency remains constant.

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