| 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. |
