Decibel:  Unit of sound level, if P1 & P2 are two amounts of power, the first is said to be n decibels greater, where n = 10 log10 (P1/P2) 
Density:  The mass of a substance per unit volume. 
Diffraction:  The bending of light around the corners of an object.

Dioptre:  Unit of power of a lens. 
Direct current:  An electrical current which always flows in one direction. 
Dispersion:  The splitting of white light into its component colors. 
Displacement:  The shortest distance between the initial and final position of a moving body. It is a vector quantity. 
Distance:  The actual path length covered by a body. It is a scalar quantity. 
Doppler Effect:  The apparent change in the frequency of a wave due to relative motion between the source and the observer. 
