Why do waves slow down at boundaries?

Waves slow down at boundaries due to a change in the medium, which affects their speed and direction.

When a wave encounters a boundary, it experiences a change in the medium through which it is travelling. This change in medium can be from one state of matter to another, such as from air to water, or from one density or temperature to another within the same state of matter. The speed of a wave is determined by the properties of the medium through which it travels, including its density and elasticity. Therefore, when the medium changes, so does the speed of the wave.

The slowing down of waves at boundaries is a result of the wave's interaction with the new medium. As the wave enters the new medium, it begins to interact with the particles of that medium. If the new medium is denser or less elastic than the previous one, the wave will take more time to move the particles and propagate through the medium, thus slowing down. This is known as refraction, which is the change in direction of a wave due to a change in its speed.

Moreover, the angle at which the wave hits the boundary also plays a role in its speed. If the wave hits the boundary at an angle, it will begin to enter the new medium at different times along its front. This causes the wave to change direction, or refract, and can also cause it to slow down.

In addition, the wave can also be partially reflected at the boundary, meaning some of the wave's energy is sent back into the original medium. This reflection also contributes to the overall slowing down of the wave at the boundary.

In conclusion, the slowing down of waves at boundaries is a complex process involving changes in medium, refraction, and reflection. Understanding this process is crucial for understanding many phenomena in physics, such as the bending of light as it passes through a lens, or the echo of sound in a room.