aberration
n. (in optics) a defect in the image formed by an optical device (e.g. a lens). In chromatic aberration the image has coloured fringes as a result of the different extent to which light of different ...
absolute space
Space that exists as a background to events and processes and is not affected by objects or other entities in the universe. The idea underpins Newtonian physics, although many physicists have always ...
absolute time
Time that exists independently of any events or processes in the universe. Like absolute space, absolute time is a basic concept in Newtonian physics.
acceptor
A substance that is added as an impurity to a semiconductor because of its ability to accept electrons from the valence bands, causing p-type conduction by the mobile positive holes left.
acceptor levels
Energy levels of an acceptor atom in a semiconductor, such as aluminium, in silicon. These energy levels are very near the top of the valence band, and therefore cause p-type conduction. See also ...
acoustoelectronic devices
Devices in which electronic signals are converted into acoustic waves. Acoustoelectronic devices are used in constructing delay lines and also in converting digital data from computers for ...
actinometer
Any of various instruments for measuring the intensity of electromagnetic radiation. Modern actinometers use the photoelectric effect; earlier instruments depended either on the fluorescence produced ...
active device
1. An electronic component, such as a transistor, that is capable of amplification.2. An artificial satellite that receives information and retransmits it after amplification.3. A radar device that ...
aerial
The part of a radio or television system from which radio waves are transmitted into the atmosphere or space (transmitting aerial) or by which they are received ( receiving aerial). A directional or ...
