In single-slit diffraction, the first minimum occurs when the light waves from different parts of the slit interfere destructively.
Condition:
The first minimum appears at an angle where the path difference between light from the top and bottom of the slit equals one full wavelength.
In simple terms (without using equations):
- Imagine dividing the slit into two equal halves.
- Light from the top half and bottom half reaches a point on the screen where their waves are out of phase.
- This means the crest of one wave overlaps with the trough of another, canceling each other.
- This destructive interference causes darkness or a minimum on either side of the central bright fringe.
This marks the first dark fringe on both sides of the central maximum in the diffraction pattern.