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I've been trying to understand why light can both be a wave and travel in a straight line, and a lot of the answers seem to include reference to the fact that the wavelength of visible light is extremely small and because of that we "don't see interference coming into play". I'm still really confused about why interference matters, but that made me consider the classic analogy for the interference pattern of light, where there are two slits on a water pond and someone creates some waves that travel through both the slits which ends up creating the pattern. What I want to know is if you could have "laser waves" on the surface of such a pond. Does a "water wave" or "water surface" analogy like this exist for lasers? If it does, what does it look like? If it does exist, how big of a pond would you need before you start seeing these effects? (Ocean size or bigger?)

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  • $\begingroup$ Laser-based versions of this seems relevant. $\endgroup$
    – J.G.
    Sep 5, 2021 at 19:26
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    $\begingroup$ You might be interested in this YouTube video: Lesson 2 - Water Waves - Diffraction, starting at time-stamp 1:08. For perspective, notice that the video uses an aperture-to-wavelength ratio of only about $5$:$1$, while a typical hand-held laser has an aperture-to-wavelength ratio of more than $1000$:$1$. $\endgroup$ Sep 5, 2021 at 19:44
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    $\begingroup$ Perhaps I'm misunderstanding your question, but there's a difference between the direction of oscillation and the direction of propagation of the wave. Think about a water wave: the water molecules oscillate up and down (very roughly speaking of course) while the motion is in a direction perpendicular (waves at sea oscillate up and down, but we still have water coming from the sea to the shore and back). Similar story with electromagnetic waves (light) $\endgroup$
    – peek-a-boo
    Sep 5, 2021 at 19:46

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You can find examples online by searching for "water diffraction", although most images have slits too narrow (compared to the wavelength) to have an obvious beam on the other side.

Here's a nice image where you can clearly see the beam:

The physics behind this is the same as the physics behind the propagation of laser beams.

See also this video (from Chiral Anomaly's comment).

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