How does the wavelength of a water wave affect diffraction?

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Discussion Overview

The discussion centers on the relationship between the wavelength of water waves and their diffraction behavior, particularly how wavelength affects the bending of waves around corners and through apertures. Participants explore theoretical aspects of diffraction, including the implications of varying wavelengths while keeping the gap size constant.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants question whether wavelength is defined for a single wave and how it influences diffraction when a wave approaches a corner.
  • One participant suggests that a single crest can be Fourier expanded into multiple crest waves, indicating that each component diffracts according to its wavelength.
  • There is a discussion about the relationship between the distance between crests and the degree of bending as waves leave a gap, with some expressing skepticism about this connection.
  • A participant mentions that when describing geometry in units of 'a wavelength', the absolute size of the gap is less relevant than the ratio of gap size to wavelength.
  • A video is shared that demonstrates changing the wavelength of a water wave through an aperture, which some participants find helpful.
  • One participant notes that transitioning from a narrow to a wide aperture allows more total energy to pass through, although the amount of energy diffracted off-axis may not change significantly.
  • There is a mention of practical applications of diffraction considerations in stealth technology, highlighting the relevance of diffraction even with larger objects.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between wavelength and diffraction, with some questioning established illustrations and others providing clarifications. The discussion remains unresolved regarding the precise influence of wavelength on diffraction in various scenarios.

Contextual Notes

Some assumptions about the definitions of wavelength and the nature of wave packets are not fully explored. The discussion also touches on the implications of diffraction in practical applications, which may require further elaboration.

ranger275
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If I send a single wave toward a corner won't it bend around the corner? If so, how does the wavelength of that single wave (is wavelength defined for a single wave?) effect how much it bends?

Every picture/illustration I have found on diffraction shows a series of lines (plane waves) approaching a gap and shows how the waves bend more if the gap is made smaller. What I haven't seen is a picture keeping the gap the same and changing the wavelength. I don't see how the distance between crests has anything to do with how much each crest bends as it leaves the gap.
 
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What do you mean by a single wave? do you mean a single crest? If so than you have to understand that short wave packets such as your single crest wave can be Fourier expanded into a sum of multiple crest waves (Principle of superposition). Each component of that expansion will diffract according with its wavelength.
 
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ranger275 said:
I don't see how the distance between crests has anything to do with how much each crest bends as it leaves the gap.

Interesting that it does, isn't it?
 
ranger275 said:
If I send a single wave toward a corner won't it bend around the corner? If so, how does the wavelength of that single wave (is wavelength defined for a single wave?) effect how much it bends?

Every picture/illustration I have found on diffraction shows a series of lines (plane waves) approaching a gap and shows how the waves bend more if the gap is made smaller. What I haven't seen is a picture keeping the gap the same and changing the wavelength. I don't see how the distance between crests has anything to do with how much each crest bends as it leaves the gap.

What is the difference? If the geometry is described in units of 'a wavelength', the absolute size of things is not relevant. The scale of the picture that's drawn may alter but the angles don't, for a given ratio of gap to wavelength.
 
ranger275 said:
What I haven't seen is a picture keeping the gap the same and changing the wavelength.

Here you go. This video shows changing the wavelength of a water wave through an aperture.

 
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Drakkith said:
Here you go. This video shows changing the wavelength of a water wave through an aperture.



Great video! (Although some of the comments below it appear to be wrong.)
 
Last edited by a moderator:
olivermsun said:
Great video! (Although some of the comments below it appear to be wrong.)

It's youtube. I'd be surprised if you didn't find a reference to hitler or nazis in the comments.
 
One point that's easy to forget. If you go from a narrow to a wide aperture, for the same wavelength, you get more total energy passing through. The actual amount of energy flowing 'off axis' may not change significantly. It's just that more energy gets through and that energy goes in the forward direction.
You get very little light through a pinhole, compared with a porthole but the porthole is still diffracting some. The diffraction at the edges can be very relevant even when an object is very large. It must, for instance, be a consideration for Stealth designs, where they go to a lot of trouble to make sure that specular reflections are angled away from the horizontal - yet there will be some energy reflected (due to diffraction) right back at the transmitter, unless they eliminate / reduce it in some way.
 
Thank you for all the comments. The video did clearly show the change in refraction as you changed wavelength.
 

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