Diffraction - the spreading of a wave past an aperture or an obstacle

Diffraction grating - a series of very many and very narrow slits

Diode - a device that lets current through it only in one direction

Diode bridge rectifier - a circuit that achieves full-wave rectification

Dipole - a pair of two equal and opposite electric charges

Direct current (dc) - current in which electrons move in the same direction with the same average drift speed

Discrete - energy that can take a set of specific values as opposed to a continuous range of values

Dispersion - the dependence of refractive index on wavelength

Displacement - the change in position; for an oscillation, the difference between the position of a particle and its equilibrium position

Distance - the length of the path followed by a particle or object