Look at the crystal clear waters in upper Ennerdale. They are deeper than they look. The light coming up from the stones at the bottom speeds up as it enters the air, which is optically less dense than water. It therefore bends away from the normal line. Our brains cannot cope with the fact that light has bent. We think it has gone straight. The diagram below shows with an X where we think the fish would be - not as deep as it actually is. The water will be deeper by a factor dependent on the amount of bending. This is called the refractive index. The refractive index of water is 1.33 so the water will be 33% deeper than it looks.
