D

D lines - two bright yellow lines in the spectrum of sodium
d.c - direct current – current that flows in one direction only, as from a battery.
damping - method of reducing the vibration of an object. (e.g. by air or electromagnetic)
dark matter - matter that is not visible but could be up to 90% of the mass of the Universe
daughter product - any nuclide that is the result of a radioactive decay
de Broglie wavelength - wavelength of a particle
decade - in units of ten, as in a 'decade resistance box'
decay - break down, get less
decay constant - a constant that determines the half life of a radioactive source
deceleration - slowing down, negative acceleration
deci - one tenth as in decimetre (0.1 m)
decibel - unit for measuring the loudness of sounds (one tenth of a bel)
declination - the equivalent of latitude on the celestial sphere. Defines position.
deflection tube - vacuum tube used to show deflection of electrons by electric or magnetic fields
Deimos - one of the moons of Mars
degrees of freedom - number of possible ways of movement of a molecule (e.g. vibration etc.)
demagnetise - remove the magnetism, make unmagnetised
Deneb - bright star in the constellation Cygnus. Luminosity = 70 000 Suns!
density - mass/volume
dependent variable - variable whose value depends on another variable that is being changed
depletion layer - region at the junction of a p-n diode where there is a net charge
depth of field - greatest distance between positions of an object to get a clear image
depth of focus - greatest distance that a film can be moved in a camera without spoiling the image
destructive interference - interference where the two waves overlap and cancel each other
deuterium - a 'heavy' isotope of hydrogen, the nucleus contains one proton and one neutron
deuteron - the nucleus of deuterium, one proton and one neutron
dewar flask - another name for a vacuum flask - after in inventor, James Dewar
dew point - temperature of air at which dew can form
diamond - crystalline form of carbon
diaphragm - in a camera the device that controls the amount of light entering the camera
diatonic - scale of eight notes to an octave, white notes on a piano
diffraction - the bending of a wave round an obstacle or its spreading through a gap
diffuse - reflection where the incident rays are reflected in all directions
diffusion - mixing of two substances by the movement of their molecules
digital - signal composed of a series of numbers, usually binary or hexadecimal
dimension - size
dimension - a method of describing quantities in terms of mass, length, time and charge
diode - a device that only allows electricity to pass through it in one direction
Dione - largest satellite of Saturn
dioptre - measurement of the power of a lens or mirror. Inverse of the focal length in metres
dip - the angle between the direction of the Earth's magnetic field and the horizontal
dipole - two equal and opposite charges a small distance apart
discharge - a breakdown of a gas (often air) allowing charge to flow from one place to another
discharge tube - a tube containing low pressure gas that is giving out light due to ionsation
dislocation - defect in a metal crystal
dispersion - splitting of white light into colours as in prism or diffraction grating
displaced - volume taken up by something, as in 'amount of water displaced by an object'
displaced - moved from one place to another
displacement - change in position. Vector version of distance.
distribution - spread. As in 'distribution of molecular speeds'.
diurnal - daily, term used in astronomy
diverging - spreading out as with a beam of light in 'diverging lens', 'diverging mirror'
domain - small area or volume, as in magnetic domains in iron
doping - the addition of impurities to a semiconductor to change its electrical properties
Doppler broadening - the broadening of spectral lines due to motion of the emitting atoms
Doppler effect - the change of observed frequency if the source or observer is moving relatively
double beam tube - electron tube filled with low pressure helium gas and with two electron guns
Double Cluster - a pair of fine open clusters in Perseus
doublet - a pair of closely spaced lines in a spectrum
drift speed - the speed of drift of electrons along a wire in an electric current
dry ice - popular name for solid carbon dioxide
dual - double, two, as in 'dual beam oscilloscope' - one with two beams
ductile - material that can be pulled out to form a wire
Dumbbell nebula - a large planetary nebula in Vulpecula (M27). Diameter 6'.
dynamics - the study of the motion of particles and bodies acted on by forces
dynamo - device for generating electricity, converts mechanical energy to electrical
dynamometer - another name for a force meter