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Why is it that an underwater observer can see only a circular "window" and also can't see anything above the separating surface? Does the "window" depend on the depth?

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  • $\begingroup$ This video shows what it looks like in a swimming pool: youtube.com/watch?v=FG6ryu0-C5w. Notice that the "window" resembles a fisheye lens: everything above water is visible, the field of view is 180° $\endgroup$ – jkien Mar 6 at 23:10
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Beyond a certain angle total internal reflection occurs at the water-air interface. This is because Snell's law $\sin \theta_{air} = n \sin \theta_{water} $ has no solution for $\theta_{air} $ if $n \sin \theta_{water} >1$ or $\theta_{water} >\sin^{-1}(1/n) $.

If you are under water you will see a disk shaped area above you in which there is the image of the hemisphere above the surface.

At large depth this light fades because of absorption and scattering.

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  • $\begingroup$ Yes. And at the critical angle, one can see the horizon (in principle, ignoring waves, low intensity). Beyond that angle, one see the reflection. $\endgroup$ – Pieter Mar 6 at 21:48
  • $\begingroup$ @my2cts Answer might be correct, but I came here looking for the exact same question...with respect to depth. So how does this answer talk about depth I was researching Snell's Window and the 200 meter light boundary to see if there was a connection. $\endgroup$ – Christopher Rucinski Jun 21 at 12:32

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