When light travelling through the air enters a transparent medium, say a sheet of glass or a pool of clear water, it
appears to bend. This is called refraction. The effect of refraction is familiar to most. A stick will appear bent
towards the surface when dipped into water (Figure 2.1a and b). Similarly, the bottom of a swimming pool
always seems closer to the surface than it really is. Kingfishers and other birds that catch fish by diving into
rivers must allow for this effect and aim below the object that they apparently see in order to hit the target
(Figure 2.1c). An equally complex problem is faced by Archer fish. These animals capture insect prey by
spitting a jet of water onto overhanging vegetation when an insect is present, knocking the prey into the water
below. The eyes of the Archer fish remain below the water during a capture attempt, and refraction as well as
associated colour changes (Section 2.6) must all be included in the ‘equation’ for the water jet trajectory.