Decibel (dB)
A bel is a unit for comparing the levels of sounds and a decibel is a tenth of a bel. If P1 and P2 are two power levels then 1dB = 10 log (P2/P1) dB.
Density
Mass per unit of volume and is measured in Kg/m
Dewar Flask
A double-walled vessel with a vacuum between the walls to reduce the transfer of heat. The inner vessel is usually made of glass as it is a poor conductor of heat and has a reflective costing that prevents radiation. The dewar flask is now traded as the Thermos flask. It was invented by James Dewar.
Dielectric
An insulator, which means that it does not allow current to flow through it. The material between the plates of a capacitor.
Diffraction
When light passes by an object or through an opening it creates a interference pattern, due to the wave nature of light.
Displacement
Is the distance represented as a vector. The SI unit is the metre.