When we look into a swimming pool, things don’t look quite as they are. In fact, it appears that everything is a little squished and that the pool itself is shallower than it should be. The reason for this has to do with how light travels through water and air.
Light coming from a source or bouncing off an object moves outward in a wave. Just as a water wave on the surface of a pool propagates outward in an expanding circle, a light wave (or any other wave moving through three dimensions) propagates outward in an expanding sphere. When the sphere of light encounters an object or surface, it illuminates it. When it passes through into another medium, such as when light leaves water and enters air, something happens to the wave.
The Nature of Light
The identity of a light wave goes deep into the realm of quantum mechanics, but for our purposes we can think of it as waves of electric and magnetic fields. Whenever an electric field moves, it creates a magnetic field, and the magnetic field in turn creates more electric fields. Together, the two produce a wave of energy that we perceive as light. This is why light is also referred to as electromagnetic radiation.
Light and Mediums
Light travels at a fixed speed through a vacuum, the speed of light. However, when light travels through a medium (such as air, water or glass), it moves a bit slower. Since light is composed of electric and magnetic fields, electrically charged particles feel its presence. As the light waves pass through, they oscillate all the protons and electrons in the medium a little, and this oscillation causes the particles to emit their own electromagnetic radiation slightly behind the original light. When these delayed waves add up, it results in a light wave with the same frequency but a shorter wavelength, producing the effect of the light moving more slowly through the medium.
As a light wave passes into a new medium, it changes speed. This change in speed is accompanied by a change in angle. A classic analogy for this is a marching band. Imagine the effect on the rows of musicians as they pass from smooth pavement into a mud bog, slowing the marchers down. The transition is at an angle, so that the left side of the group hits the mud first, followed by the right. As you can imagine, if the slower marchers on the left want to stay in line with the faster ones on the right, they all have to change direction slightly. Once they’re all in the mud, they are moving a little to the left of their original heading. In the same way, a wave front of light changes direction as it passes from one medium to another.
In this way, refraction of light produces the shallow swimming pool illusion. As you look down at a pool, you are seeing light that has changed angle as it moves out of the water and into the air. Specifically, light moves faster through air than water. Light coming up from the bottom of the pool refracts down to arrive at your eyes at a shallower angle. You see the bottom of the pool a little higher and farther away than it actually is.