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This question should be a simple one, but I find conflicting infromation from different sources. At what aperture size is the diffraction of a water (or sound) wave the greatest? In my textbook, it has been said that:

Waves are diffracted when they pass through a gap or around the edge of an obstance. The effect is biggest when the width of the gap is equal to the wavelength of the ripples.

However, a Quora answer says something else:

[...] it's at a maximum (an outgoing spherical wave) for an infinitesimal pinhole. Google “Huygens's Principle”.

This stance is supported by another website:

When the gap size is smaller than the wavelength, more diffraction occurs.

Which is it? Is diffraction the greatest for an infinitesimally small aperture or for an aperture with the same width as the wavelength of the wave?


Note: Being a high school sophomore, I am not interested (because I may not understand) why this happens for light and in other non-GCSE level answers.

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  • $\begingroup$ Similar questions exist on Physics SE (I did see them), but they either don't have an answer, or don't answer the question. $\endgroup$
    – Shane
    Nov 13 '21 at 5:39
  • $\begingroup$ Huygens principle is about light. The wavelength of water waves and electromagnetic waves is very different, so there' is no conflict. $\endgroup$
    – Alchimista
    Nov 13 '21 at 9:01
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For water and sound waves, the greatest amount of diffraction will occur when the size of the aperture is about the same as the wavelength of the wave. If the aperture is very small compared to the wavelength, a majority of the wave will be blocked. If the aperture size is large compared to the wavelength, the wave will pass through without any significant diffraction.

Seems like

it's at a maximum (an outgoing spherical wave) for an infinitesimal pinhole

is talking about light, which may be causing the confusion.

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