Cycle

　　In oscillation, a cycle occurs when an object undergoing oscillatory motion completes a “round-trip.” For instance, a pendulum bob released at angle has completed one cycle when it swings to and then back to again. In period motion, a cycle is the sequence through which a system once during each oscil-lation. A cycle can consist of one trip up and down for a piece of stretched string, or of a compression followed by a rarefaction of air pressure for sound waves.

　　De Broglie wavelength

　　A wavelength, given by = h/mv, which is associated with matter. Louis de Broglie proposed the idea that matter could be treated as waves in 1923 and applied this theory successfully to small particles like electrons.

　　Decay constant

　　A constant, , not to be confused with wavelength, that defines the speed at which a radioactive element undergoes decay. The greater is, the faster the element decays.

　　Decibel

　　A logorithmic unit for measuring the volume of sound, which is the square of the amplitude of sound waves.

　　Deposition

　　The process by which a gas turns directly into a solid because it cannot exist as a liquid at certain pressures.

　　Destructive interference

　　The cancellation of one wave by another wave that is exactly out of phase with the first. Despite the dramatic name of this phenomenon, nothing is “destroyed” by this interference—the two waves emerge intact once they have passed each other.

　　Diffraction

　　The bending of light at the corners of objects or as it passes through narrow slits or apertures.

　　Diffraction grating

　　A sheet, film, or screen with a pattern of equally spaced slits. Typically the width of the slits and space between them is chosen to generate a particular diffraction pattern.