The apparent depth of a pool depends on the angle between the normal to the pool surface and the ray that reaches the observer’s eye. For small angles, the ratio of apparent to actual depth is , where is the index of refraction of the water. As the angle increases, the image of a point on the bottom becomes astigmatic giving rise to two principal images. For both images, the apparent depth becomes smaller as the angle becomes larger. I show that the appearance of a typical gymnasium pool is significantly different depending on whether the observer is standing at the shallow or the deep end of the pool. In either case, one of the principal astigmatic images will be much more apparent than the other.
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