Refracted Wave: Definition & Overview

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

This lesson will explain what a refracted wave is, and how it works. We will also discuss why exactly it happens in the context of wave speed. A short quiz will follow.

What is a Refracted Wave?

Have you ever put a spoon in a glass of water? When you look at the surface, what do you notice? If you look from the right angle, it appears as though the spoon is in two pieces!

Refraction of a Spoon
Refraction of a Spoon

But not to worry -- your spoon is perfectly fine. You've just fallen victim to the physics of refraction!

When you shine a beam of light from a material of one density into a material with a different density, that light will bend. That bending is called refraction, and the light ray that has bent is called a refracted ray. Since light is a wave, it would be just as valid to instead call it the refracted wave. Refraction happens with light, but it also happens with all other kinds of electromagnetic waves: radio waves, microwaves, infrared, ultraviolet, x-rays and gamma rays. It's just easier to see with visible light.

Refraction in a Glass Block
Refraction in a Glass Block

Back to the spoon! When light hits the spoon inside the water, it has to leave the water before it gets to your eyes. When it goes from the water to the air, this light refracts (bends), and this makes the underwater part of the spoon appear to be in a different place to where it actually is.

But how do you predict which way the light will bend?

It's all about the density of the materials. If the light wave moves from a less dense place to a denser place, it will bend towards the 'normal'. The normal is an imaginary dotted line that points at 90 degrees to the surface (see the diagram below). On the other hand, if the light wave goes from a higher density material into a lower density material, the wave will bend away from the normal.

A Refracted Wave Bending in a new Material

Why Does it Happen?

It's a sunny day and you decide to mow the lawn. At first you push the mower down your driveway, and it moves very easily. But when you push it onto the rough, long grass of your lawn, you have to push much harder.

Now imagine for a second that you went from your driveway onto the lawn at an angle. The corner of the lawnmower that got to the grass first would slow down first, but the corner that was still on the driveway would keep moving just as easily. Because of this, the lawnmower would bend -- the corner still on the driveway would swing around, changing the direction the lawnmower is facing. This is like what happens with refraction, just with a wave instead of a lawnmower.

Lawn Mower Refraction Because of Dense Grass
Lawn Mower Refraction Because of Dense Grass

The speed of light is different in different materials. Light moves slower in dense materials and faster in less dense materials. So when one side of the light wave hits a new material before the other, the wave will bend. If the new material is denser, the corner that hits it first will slow down, and the wave will bend towards the normal. If the new material is less dense, the corner that hits it first will speed up, and the wave will bend away from the normal.

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