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Sunlight is incident on a pinhole of diameter 0.1 micrometre, producing a diffraction pattern. If the pinhole diameter is slightly increased, how does the diffraction pattern change?

1. Its size decreases and its intensity decreases
2. Its size increases and its intensity increases
3. Its size increases but its intensity decreases
4. Its size decreases but its intensity increases

Correct answer: Its size decreases but its intensity increases

Solution

The angular width of the diffraction pattern from an aperture varies as lambda/D, so increasing the diameter D reduces the angular spread, i.e. the pattern shrinks in size. At the same time a larger hole lets through more light energy, so the central spot becomes brighter (intensity increases). Therefore the size decreases but the intensity increases.

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