JEE Main 2021 · 25 February · evening · Single correct · +4 / −1
Consider the diffraction pattern obtained from the sunlight incident on a pinhole of diameter 0.1 μm. If the diameter of the pinhole is slightly increased, it will affect the diffraction pattern such that:
- (a) its size increases, but intensity decreases
- (b) its size increases, and intensity increases
- (c) its size decreases, but intensity increases
- (d) its sizes decreases, and intensity decreases
Answer and solution
Answer (c) its size decreases, but intensity increases
Solution
Physics sir · 4 steps
A wider hole spreads light less and lets more light in, so the pattern shrinks and gets brighter.
- Angular spread. The central bright disc of a circular aperture spreads out to the first dark ring, at an angle set by the wavelength and the hole diameter d.
sin θ ≈ (1.22 λ)/d
- Size of pattern. θ varies as 1/d. Increase d and θ falls, so the diffraction pattern on the screen becomes smaller.
- Intensity. A larger hole has a larger area, so more light energy passes through it. That greater energy also falls on a smaller patch of the screen, so the intensity increases.
- So. The size decreases and the intensity increases, which is option (c).
The idea to keep. Diffraction spread goes as λ/d: a bigger opening means less spreading. More area lets in more energy, and a tighter pattern packs it closer, so brightness rises.
Answer from the NTA final answer key. Solution by PrepWiser.