R

radar - Radio Detection And Ranging - means of detecting an object by wave reflection radial - along a radius
radial velocity - velocity along a radius (towards or away from) with the Earth at the centre
radian - unit of angle 57.296^o. 2p radians = 360^o.
radiant - point in the sky from which meteors in a shower seem to come from
radiation - energy transmitted by radiant energy, as in 'heat radiation'
radioactivity - spontaneous disintegration of nuclei
radiometer - device for detecting heat radiation, as in Crookes radiometer
radio telescope - a telescope that detects radio waves from the universe
rainbow - bow shaped spectrum due to refraction of sunlight in rainwater droplets
RAM - random access memory
ramsden - type of telescope eyepiece with a flat, but fairly narrow, field of view
random - in all directions as in 'random motion'
random error - one due to poor experimental technique
range - of a projectile, distance between point of launch and point of return to ground
ray - a thin 'line' of light
ray box - a simple box containing a light bulb used in optics, accessories - slits and a lens
reactance - the a.c resistance of a component such as an inductor or capacitor
reaction - an opposite force to the applied force (action)
reactor - a place where a reaction takes place, as in a 'fission reactor' - fission takes place
recall - remember and state
recession - movement away, as in 'recession velocity'
rectifier - a device that converts a.c to d.c - only allows current through in one direction
red giant - large, 'cool', reddish star. Surface temperature around 3000 ^oC
red shift - reddening of light from a galaxy due to Doppler shift for a receding object
reed relay - a relay that operates using a vibrating metal reed (reed switch)
reflection - bouncing of a wave (such as light) off a surface
reflector - type of telescope with a mirror as the objective
refraction - bending of a wave when it passes from one material to another
refractive index - sin i/sin r for light incident on a transparent material. High values refract more.
refractor - type of telescope with a lens as the objective
regelation - the change in melting point due to pressure
relation - a connection between two or more quantities
relationship - a connection between two or more quantities
relative - compared with
relative velocity - the velocity of one object compared with another object
relativity - a theory developed by Einstein: motion is relative, constancy of the speed of light
relay - a device where one circuit controls another. Low current controlling a large one.
resistance - a property of a material that reduces current flow through a circuit
resistance thermometer - a thermometer that measures temperature by a change of resistance
resistivity (r) - the resistance across two faces of a unit cube of the material. r = RA/L
resistor - a component that reduces current flow through a circuit
resolution - separation of two images that are close together
resolution - splitting a vector into two components at right angles
resolve - separate
resolving power - the ability of a telescope, microscope of the eye to resolve two images
resonance - large amplitude when the driving frequency is the same as the natural frequency
rest mass - the mass of a body as measured by an observer that is at rest relative to it
resultant - vector formed by the addition of two or more other vectors, e.g. resultant force
retina - light sensitive surface on the inner side of the eyeball
retrograde - backwards motion, as of a planet in an orbit
reverberation - an audible sound that lasts after the energy input has stopped
reverberation time - time for the sound level to fall to one millionth of the original (1- 2.5s is good)
rheostat - variable resistor
Richter scale - scale on which the strengths of earthquakes are measured
Rigel - hot, bright star in Orion. High absolute magnitude. As bright as 60 000 Suns
right ascension - the equivalent of longitude on the celestial sphere
rigid body - a body where the distance between all macroscopic particles remains the same
rille - elongated lunar valley
ring nebula - a bright ring nebula in Lyra 1 minute of arc in diameter (M57) - discovered 1779
ripple tank - apparatus for showing wave motion
rocket - a projectile powered by ejecting gas. Carriers its own oxidant with it.
rods - very sensitive retina nerve cells . Good at low intensity but do not detect colour.
roentgen - unit of X ray dose
ROM - read only memory
root mean square - square root of the sum of the mean squares of a quantity (r.m.s.)
r.m.s velocity - square root of the mean square velocities of a group of gas molecules
rotation - spin
rotating - turning or spinning
rotor - rotating part of an electric motor, generator or dynamo