Stationary phase approximation

The stationary phase approximation is a way of making calculations simpler when you have to do a lot of math. It works best when you have a fairly simple case, like when you are looking at a wave that is either going up and down or side-to-side.

Imagine a wave like a line going up and down. It goes from a high value to a low value, then back up again, and so on. When you use the stationary phase approximation you only look at the points where the wave has stopped moving and has reached a "stationary phase". These points are called "extrema" and they are the highest points (top of the wave) and the lowest points (bottom of the wave).

The stationary phase approximation lets you find the height of the wave without having to do a lot of complicated math. This can be very useful when you have to think about a lot of different waves or when you have to make calculations quickly.