Phase velocity

Phase velocity is like playing a game of 'follow the leader' with waves. Imagine you and your friends are standing in a line holding hands, and you pass a ball down the line. You don't want to lose the ball, so you have to pass it along fast enough that it doesn't fall to the ground.

Waves are kind of like the ball in this example. They're made up of tiny particles moving up and down really quickly. When a wave passes through a material, they're like your friends in the line. The wave's particles pass the energy from one to another, so the wave moves along. This speed of the wave is called the phase velocity.

Now imagine that you and your friends are playing this game on a giant bouncy castle. If you run too fast, you might actually bounce the ball off the bouncy castle and lose it. So, to keep the ball under control, you have to slow down a bit. Likewise, when waves pass through different materials (like from air to water), the phase velocity can change. The speed of the wave changes because of different properties of the material.

So, phase velocity is like the speed at which waves move through different materials. It's like playing 'follow the leader' and passing a ball down a line, where the speed at which you pass the ball changes depending on which material you're passing it through.