Damping – a frictional force (which will cause motion to cease if there are no forces)

de Broglie wavelength – a characteristic wavelength for a moving particle; Planck's constant divided by the particle's momentum (see equation 28.2)

Decibel – unit of measurement of sound intensity

Density – the mass of an object per unit volume

Destructive interference – the decrease in amplitudes when two waves, which are out of phase, are added

Dielectric – electrically nonconducting material

Dielectric constant – - the ratio of capacitance with a dielectric between the plates to the capacitance with a vacuum between the plates (see equation 17.13)

Dielectric strength – the maximum electric field that a material can withstand before breaking down or losing its ability to insulate

Diffraction- the bending of light as it passes through a small aperture or around a sharp edge

Diffraction grating – a large number of parallel, equally spaced slits

Dimensions – any measurable quantity where the numerical result depends on the units chosen for measurement

Diode – an electronic device that allows current to flow more easily in one direction than in the other direction

Diopter – the unit of power (strength) for a lens; the reciprocal of the focal length, in meters (see equation 23.1)

Dipole moment – for a dipole (a positive charge and a negative charge separated by a small distance) a vector directed away from the negative to the positive charge whose magnitude is the product of the charge and the separation

Direct current – electric current in which the charge always flows in the same direction

Dispersion – the spreading of light because of the wavelength dependence of the index of refraction

Displacement – the change in position of an object with regard to the origin of the coordinate system being used.

Dissipative forces – forces that remove mechanical energy from a system (like friction)

Diverging lens – lens that refracts incident light away from the optical axis

Doping – the controlled addition of impurities to a semiconductor

Doppler effect – change in frequency caused by relative motion between sound source and a listener (see figure 15.11)

Dose – the energy deposited per unit mass by absorbed radiation

Dose equivalent – the product of the dose times a number (quality factor) which takes into account the biological effect of each radiation

Drag coefficient – a constant related to air resistance of a moving object

Dynamic equilibrium – that state of motion in which the velocity is constant

Dynamics – the study of the causes of motion