The name given to this behaviour is refraction.

Quick Scoop

When waves pass from one medium to another (for example, from deep water to shallow water or from air to glass), their speed changes, and this causes them to bend or change direction.

This bending due to a change in speed is what physics calls refraction.

Simple way to picture it

  • Imagine a line of marching soldiers moving from a smooth road onto mud at an angle. The first foot that hits the mud slows down, while the other foot is still on the road and moving faster, so the line swings around.
    Waves do the same thing when they slow down in a new medium: the change in speed makes them turn, and that turning is refraction.

So, “when waves slow down they change direction” is describing the refraction of waves. ✅

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