While we think of the speed of light as a constant (186,282 miles per second), that is its speed in a vacuum. When light passes through air, water, or other substances, it travels slower than that. Its speed varies, depending on what it is traveling through. It travels much slower in water than it does in air.

When its speed changes as it moves from one substance to another at any angle other than 90° or 0°, then its path will be bent. This is called refraction, and it applies to all waves, not just light. Sound waves are refracted when they pass through a balloon filled with carbon dioxide, slowing them down slightly. Waves in water are refracted when the water changes depth, which changes the speed of the wave. A sand bar offshore can cause an easily visible change in the direction of the waves.