The wave nature of light leads to two very important properties: refraction, where the direction of light propagation is altered at the boundary between media of different densities, and diffraction, which has among its consequences that light can "bend around corners".
|Refraction of light|
The apparent and actual positions of the fish differ because the direction of light propagation has been changed as light passes from the more dense water into the less dense air.
If we adopt the convention that the light passes from medium 1 into medium 2, the general rule is that the refraction is
Diffraction has a number of consequences for astronomy. Two of the more important are that this property is the basis for the diffraction grating that can be used to separate light into its constituent colors, and that diffractive effects set an absolute limit on the quality of an image observed through an optical instrument such as a telescope. This diffractive limit occurs because the lenses of such objects are of finite size and diffract light because they cut off part of the light wave.