Why does wave speed change when transitioning between media?

Wave speed changes when transitioning between media due to differences in the media's physical properties, such as density and elasticity.

In more detail, the speed of a wave is determined by the medium through which it travels. This is because waves are disturbances that propagate through a medium, and the properties of that medium can significantly affect how quickly the disturbance can move. Two key properties that influence wave speed are the medium's density and its elasticity.

Density refers to the mass per unit volume of a medium. In general, waves travel slower in denser media because the particles in the medium are closer together and thus interact more frequently. This increased interaction slows down the wave's progress. For example, sound waves travel slower in water (a denser medium) than in air (a less dense medium).

Elasticity, on the other hand, refers to a medium's ability to return to its original shape after being disturbed. Media with high elasticity, such as steel, allow waves to travel faster because the particles in the medium can quickly return to their original positions after being disturbed, allowing the wave to move on more rapidly. Conversely, in less elastic media, like rubber, waves travel slower because the particles take longer to return to their original positions.

When a wave transitions from one medium to another, it encounters a change in these physical properties. This change can cause the wave to speed up or slow down, depending on the properties of the new medium compared to the old one. This is why, for instance, light refracts (changes direction) when it passes from air into water: the change in speed caused by the transition between media alters the wave's direction of travel.

In summary, the speed of a wave is not a fixed property of the wave itself, but is instead determined by the characteristics of the medium it's travelling through. When these characteristics change, as they do when a wave transitions between different media, the wave's speed changes accordingly.