For diffraction effects to be observable, the size of the obstacle (or aperture) should be:

of the same order as the wavelength
much larger than the wavelength
unrelated to the wavelength
exactly half the wavelength

Correct answer: of the same order as the wavelength

Solution

Appreciable bending of waves (diffraction) occurs only when the obstacle or aperture has a size comparable to the wavelength of the wave. If the obstacle is much larger, the wave essentially travels in straight lines and diffraction is negligible.

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