When light waves enter a different medium, they change direction (bend), and this process is called refraction. This occurs because the mediums have different densities.

Because of refraction, this spoon looks broken.

The spoon clearly hasn’t snapped in half, it is sitting in the cup as normal. However, it looks distorted because of refraction.

Light reflects off the top of the spoon, which isn’t in the water. At this part of the spoon, light is travelling through the same medium, which is air.

However, as light travels into the water to reach the bottom of the spoon, it crosses from one medium to another, from air to liquid. Because of this, light doesn’t travel in a straight line to the bottom of the spoon and again back to our eye.

When light enters the new medium, it changes direction. Light has to change direction when both entering and leaving a new medium, from air to liquid and then from liquid to air. This makes the spoon appear distorted.

When looking at refraction, we have to consider how light changes direction between two mediums.

For example, when a torch is pointed at glass.

1. As light moves from air to glass, it changes direction.

  • The reason light changes direction is that it changes speed, slowing down as it enters the glass

2. When the light leaves the glass and moves back into the air, it changes direction again.

  • This time, the light speeds up as it moves back into the air

Refraction Diagrams

Let’s look at a refraction diagram:

When using a refraction diagram, you can calculate the angle of incidence, the angle of refraction and the angle of emergence.

The angle of incidence is equal to the angle of emergence. This is because light travels in the air both before it enters the glass and after it leaves the glass.

  • When light enters a more dense medium, it slows down, so the ray bends towards the normal
  • When light enters a less dense medium, it speeds up, so the ray bends away from the normal

Keep in mind, the ‘normal’ is the dotted line in the diagram above.

So, if light enters a more dense medium, the angle of refraction is smaller and if light enters a less dense medium, the angle of refraction is greater.