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Snell’s window

Stories from Physics for 11-14 14-16 IOP RESOURCES

Anglers can hide themselves from their prey by exploiting a phenomenon arising from total internal reflection. When the surface of a pool of water is flat, a fish beneath the surface will observe the entire hemisphere above the water in a cone of angle 97°, a region known as Snell’s window or the optical man-hole. So if anglers position themselves at a low viewing angle relative to the surface of the water, their image is squashed and out of focus to the fish and they will remain undetected.

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