Huygens' Principle

The propagation of light or any wave through space can

be described by Huygens' principle. Each point on a

primary wavefront acts as the source of secondary wave

that propagating with the same speed and frequency.

Coherent Waves and Interference

A phase difference ( Df ) between two group waves is constant in time are coherent waves. Only coherent light waves interfere with each other.

Constructively and Destructively

When two light waves of the same frequency

and wavelength but different in phase combine,

the resultant wave is of constructive interference

if the phase difference is a whole number of period

0, T, 2T, 3T..., or say the path difference is

a whole number of wavelength ( 0, l, 2l, 3l, ... );

and interfere destructively if the phase difference