*Phys Postlims (198-208).qxd
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pivot (or fulcrum) The point of support about which a lever turns. plane A flat surface. polarization Of a radiated wave, being oriented in a predictable direction perpendicular to the direction of propagation. Polaroid Trade name for a transparent plastic sheet containing many minute double-refracting crystals with parallel axes, which produces plane-polarized light when ordinary light falls on it. position vector (directed line segment) It represents the displacement of a point from a reference point. potential difference The difference in electrical potential between two points on a circuit or in an electric field. potentiometer An instrument used to give an accurate measurement of e.m.f. or potential difference. power The rate of doing work or the rate at which energy is transferred. pressure The force acting per unit area. In a liquid at rest, pressure increases with depth and acts equally in all directions. primary colors Red, green, and bluish-violet. When light of these three colors is mixed, white light is formed. principal axis The line that passes through the central point of the lens. principle of superposition When a wave at its point of maximum positive amplitude meets another of equal amplitude at its point of minimum amplitude, the result is zero. If the maxima of two waves meet, the resulting wave is the sum of both waves. prism A block of transparent material with a triangular cross section. proton One of the basic particles of the atom, found in the nucleus with the neutron. It has a positive charge and is similar in mass to the neutron. p-type Semiconductor material treated to contain an excess of positive charge carriers (sometimes referred to as “holes”). pulley A simple machine for raising loads, consisting of a grooved wheel over which a rope or chain passes. pyrometer An instrument for measuring high temperatures. radiation The transmission of energy by the movement of waves and particles in all directions. It does not require a medium in which to travel. radioactivity The spontaneous disintegration of certain isotopes, accompanied by the emission of radiation. radio waves A form of electromagnetic radiation. radius A straight line joining the center of a circle to a point on its circumference. rarefaction The opposite of compression for longitudinal waves. Rayleigh criterion The criterion for the resolution of interference fringes, spectral lines, and images. Rayleigh’s scattering law When electromagnetic radiation is reflected off atoms there is a change in phase but not frequency. rectifier A device allowing electric current to flow freely in one direction only. reflecting telescope A telescope that brings light rays to a focus using a mirror. reflection The process by which radiation incident on a surface does not penetrate the surface but bounces back.
refracting telescope A telescope that brings light to a focus using lenses. refraction The change of direction of electromagnetic radiation, such as light, on passing from one medium to another. refractive index The constant representing the amount of refraction occurring when electromagnetic radiation passes from one medium to another. relay (electrical) An electromechanical switch. resistance A measure of the extent to which a material can oppose the passage of an electric current. resolving power The potential an optical instrument has for producing distinguishable images. resonance The state of a body subjected to a periodic disturbance at a frequency close to or equal to the frequency at which it naturally vibrates or oscillates. respiration The process in living organisms by which chemical energy in food is liberated. resultant The single vector representing the sum of a set of vectors. retina The light-sensitive area of the eye consisting of rod and cone cells. Richter scale A logarithmic scale used to indicate the magnitude of earthquakes. satellite A small body orbiting a larger one under the influence of gravity. scalar A quantity that has magnitude but not direction. secondary colors Magenta, cyan, and yellow. Each is formed from two different primary colors. seismic wave A shock wave produced by an earthquake or explosion. self inductance If the current in a circuit changes, the magnetic flux linked to the circuit changes and induces an e.m.f. the direction of which opposes the change causing it. semiconductor Covalent crystals the resistance of which is between that of insulators and conductors. Current is carried by free electrons, leaving positively charged holes behind. short-circuit An electrical connection of negligible resistance that bypasses a part of an electrical circuit. SI Système International (d’Unités): a system of coherent metric units used in science. simple harmonic motion Vibrational motion, in which acceleration toward a point is proportional to the distance from it. sine wave The projection on a plane surface of the graph of a point moving around a circle at uniform speed, and the graph of the function y = sin x or y = cos x. solar panel A grouping of solar cells used to collect energy from sunlight using the photovoltaic effect to create electricity by absorbing the Sun’s energy. solenoid A coil of insulated wire, cylindrical in shape, with its length longer than its diameter. sonar A system used to detect and locate objects using reflected sound waves. sonometer An apparatus that is used to study the vibrations of a wire or string. sound waves A series of alternate compressions and rarefactions (longitudinal waves) of a medium through which energy is transmitted. spectrograph An instrument that makes a photographic record, a spectrogram, of a spectrum used in analysis.
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