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Glossary R

Radar
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 analysing 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. [more]
Radian
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.
Radioactivity
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 . [more]
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
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. [more]
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