The sunlight is allowed to fall on a pinhole of diameter \( 10^{-4} \) mm and thus a diffraction pattern is obtained. How will the size and intensity of the diffraction pattern be affected, if the diameter of the pinhole is slightly increased?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → size decreases; intensity increases
In diffraction at a circular aperture of diameter \( d \), the angular radius of the central bright disc is
\( \displaystyle \theta \approx \frac{1.22\,\lambda}{d} \)
So the spread of the pattern is inversely proportional to the diameter of the hole.
Size: when \( d \) is increased, \( \theta \) becomes smaller, so the diffraction pattern shrinks.
Intensity: a wider hole lets more light through, and this larger amount of light is spread over a smaller central disc, so the pattern becomes brighter.
So the size decreases and the intensity increases.
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