The intensity of light in an interference pattern depends on how the light waves combine at a particular point, based on their phase relationship:
- Constructive Interference:
- When two light waves arrive in phase (their crests and troughs align), their electric fields add.
- This leads to an increase in amplitude, and since intensity is proportional to the square of amplitude, the resulting light is brighter.
- These points form the bright fringes in an interference pattern.
- Destructive Interference:
- When two waves arrive out of phase (crest meets trough), their electric fields cancel out.
- This results in reduced or zero amplitude, leading to low or zero intensity (dark fringes).
- Intermediate Interference:
- If waves are partially out of phase, they don’t completely cancel or reinforce.
- This gives intermediate intensity, creating a graded pattern of brightness.
Thus, interference modulates light intensity, creating a pattern of alternating bright and dark bands. The variation in intensity is a direct result of the constructive and destructive interference of the overlapping light waves.