Why do waves bend when entering a different medium?

Waves bend when entering a different medium due to a phenomenon known as refraction, caused by a change in speed.

Refraction is a fundamental concept in wave behaviour, which occurs when a wave passes from one medium to another, causing a change in its speed and subsequently, its direction. This bending of waves is most commonly observed with light waves, but it also applies to other types of waves such as sound and water waves.

The principle behind refraction is that waves travel at different speeds in different media. For instance, light travels faster in air than in water or glass. When a wave, such as a light wave, hits a new medium at an angle (not straight on), one side of the wave will slow down before the other. This causes the wave to change direction, a process we perceive as bending.

The amount of bending, or the angle of refraction, depends on the change in speed and the angle at which the wave hits the new medium. This is described by Snell's Law, a fundamental equation in optics. Snell's Law states that the ratio of the sine of the angle of incidence (the angle at which the wave hits the new medium) to the sine of the angle of refraction (the angle at which the wave bends) is equal to the ratio of the speed of light in the two media.

In essence, the greater the difference in speed between the two media, the more the wave will bend. Similarly, the more oblique the angle at which the wave hits the new medium, the more it will bend. This is why a straw appears to bend when it is placed in a glass of water, or why the sun appears distorted when viewed underwater.

Understanding refraction is crucial in many areas of physics and engineering, including the design of lenses and optical instruments, the study of seismic waves in geophysics, and the transmission of radio signals.