Water in a swimming pool or water tank appears shallower than its depth. Why ?

Explanation

When light rays obliquely from an optically denser medium ( water, in this case ) to an opticaly rarer medium( air, in this case ) , they bend away from the normal at the point of incidence. As a result, the bottom of a swimming pool or water tank appears raised to an observer standing near the edge of the pool or the tank. Therefore, the swimming pool or water tank appears shallower than its depth.