Why light bends

The idea

Refraction is due to the change in the speed of light as it enters from one transparent medium into another.

Fast and slow

Light travels very fast in air. In glass or water it moves more slowly.

What do you think is the reason that a ray of light bends when it enters glass at a slant?

• The speed of light changes in the glass
• The colour of light changes in the glass
• The glass makes the light brighter

Speed along the ray

Press Next step to follow a ray through a glass slab. The green arrow shows the speed of the light.

Slower, so it bends

In glass the arrow was shorter: the light was slower. Wherever the speed changed, the ray changed direction.

Refraction is due to the change in the speed of light as it enters from one transparent medium into another.

Notes

Refraction is due to the change in the speed of light as it goes from one transparent medium into another.

Check yourself

What is refraction due to?

Light goes from air into glass. Its speed in the glass is

A ray travels from glass back into air. Its speed

• A change in the colour of the light. Refraction is due to the change in speed. The laws of refraction hold for light of a given colour.
• The light bouncing off the surface. A bounce is reflection. Refraction happens because the speed of light changes in the second medium.
• The change in the speed of light as it enters another transparent medium — correct. Yes!
• less than in air — correct. Yes!
• more than in air. The speed of light reduces considerably in glass or water compared with air.
• the same as in air. The speed differs from medium to medium. In glass it is less than in air.
• decreases. From glass to air the light goes from a slower medium to a faster one, so it speeds up.
• increases — correct. Yes!
• stays the same. The speed of light is different in different media, so it changes on leaving the glass.