In a two-slit interference experiment, destructive interference occurs when the two light waves cancel each other out. This happens when the waves are out of phase by half a wavelength, causing the crest of one wave to meet the trough of the other.
The condition for destructive interference is:
- The path difference between the two waves must be an odd multiple of half the wavelength.
In simple terms:
- The waves must travel distances that differ by (½, 3⁄2, 5⁄2, …) wavelengths.
- This causes them to arrive exactly out of phase, leading to cancellation.
As a result, dark fringes (or minima) appear on the screen at positions where destructive interference occurs.
This condition depends on:
- The wavelength of the light,
- The distance between the slits,
- And the position on the screen where the interference pattern is observed.
Destructive interference is a key feature of the alternating dark and bright fringes seen in the interference pattern.