What should be the size of an obstacle to observe diffraction?

The correct answer is Option (3) → should be of the same order as wavelength

Diffraction is the bending of waves round the edges of an obstacle or aperture. For an opening or obstacle of size \( a \), the angle through which the wave spreads is of the order of

    \( \displaystyle \theta \approx \frac{\lambda}{a} \)

This spreading is noticeable only when \( \lambda/a \) is not small, that is, when the size of the obstacle is comparable to the wavelength.

Option (1) has no relation with the wavelength — Incorrect. The spreading \( \lambda/a \) depends on the ratio of the wavelength to the size, so the size and the wavelength are related.

Option (2) should be exactly half of the wavelength — Incorrect. No exact value is needed; any size of the same order as \( \lambda \) gives clear diffraction.

Option (3) should be of the same order as wavelength — Correct. When \( a \) is of the order of \( \lambda \), the angle \( \lambda/a \) is large enough to see.

Option (4) should be much larger than the wavelength — Incorrect. For \( a \gg \lambda \) the spreading is negligible and the wave travels almost in straight lines (ray optics).

Note: A similar question was also asked in CUET (UG) 2025 on 30-05-2025 (Slot 2).

NCERT (2026-27):
Diffraction – “… Diffraction is a general characteristic exhibited by all types of waves, be it sound waves, light waves, water waves or matter waves. Since the wavelength of light is much smaller than the dimensions of most obstacles; we do not encounter diffraction effects of light in everyday observations.”