Consider the diffraction pattern obtained from the sunlight incident on a pinhole of diameter 0.1 \mu m. If the diameter of the pinhole is slightly increased, it will affect the diffraction pattern such that:

its size decreases, but intensity increases

its size increases, but intensity decreases

its size increases, and intensity increases

its sizes decreases, and intensity decreases

Wave Optics PYQ — JEE Main 2021 · 25 Feb · Shift 2 · Q53 with Solution

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Correct answer: C

Relevant idea: diffraction from a circular aperture

For a pinhole (circular aperture), the diffraction pattern is an Airy pattern. The angular radius of the केंद्रीय bright spot is

$\theta \approx 1.22\,\frac{\lambda}{d}$

where:
- $\lambda$ = wavelength of light
- $d$ = diameter of the pinhole
Effect of increasing the pinhole diameter

From $\theta \propto \frac{1}{d}$ if $d$ is increased slightly, then $\theta$ decreases.

So the diffraction pattern becomes smaller in size.
Effect on intensity

If the diameter of the pinhole increases, the aperture area increases:

$A = \pi \left(\frac{d}{2}\right)^2 \propto d^2$

Hence more light passes through the pinhole. Therefore the diffraction pattern becomes more intense.
Check options
- A: size increases, intensity decreases $\rightarrow$ wrong
- B: size increases, intensity increases $\rightarrow$ wrong
- C: size decreases, intensity increases $\rightarrow$ correct
- D: size decreases, intensity decreases $\rightarrow$ wrong
Final answer

Increasing the pinhole diameter makes the diffraction pattern smaller and brighter.

Therefore, the correct option is C.