D

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.

Direction

The property of a vector that distinguishes it from a scalar: while scalars have only a magnitude, vectors have both a magnitude and a direction. When graphing vectors in the xy-coordinate space, direction is usually given by the angle measured counterclockwise from the x-axis to the vector.

Directly proportional

Two quantities are directly proportional if an increase in one results in a proportional increase in the other, and a decrease in one results in a proportional decrease in the other. In a formula defining a certain quantity, those quantities to which it's directly proportional will appear in the numerator.

Dispersion

The separation of different color light via refraction.

Displacement

A vector quantity, commonly denoted by the vector s, which reflects an object’s change in spatial position. The displacement vector points from the object’s starting position to the object’s current position in space. If an object is moved from point A to point B in space along path AB, the magnitude of the object’s displacement is the separation of points A and B. Note that the path an object takes to get from point A to point B does not figure when deining displacement.

Distance

A scalar quantity. If an object is moved from point A to point B in space along path AB, the distance that the object has traveled is the length of the path AB. Distance is to be contrasted with displacement, which is simply a measure of the distance between points A and B, and doesn’t take into account the path followed between A and B.

Doppler shift

Waves produced by a source that is moving with respect to the observer will seem to have a higher frequency and smaller wavelength if the motion is towards the observer, and a lower frequency and longer wavelength if the motion is away from the observer. The speed of the waves is independent of the motion of the source.

Dot product

A form of vector multiplication, where two vectors are multiplied to produce a scalar. The dot product of two vectors, A and B, is expressed by the equation A · B = AB cos .

Dynamics

The application of kinematics to understand why objects move the way they do. More precisely, dynamics is the study of how forces cause motio