Refraction

If you shine a beam of light (a bundle of parallel rays) through the air, it will travel in a straight line. Rays of light usually travel in straight lines until they hit something.

If a ray of light hits the surface of a sheet of glass, some light will be reflected by the surface of the glass. However, much of the light will pass through the glass, because glass is transparent.

Water affects light rays in a similar way. Some light will be reflected off the surface of the water, but much of the light will pass through the transparent water.

When a ray passes from air into glass the direction in which the light ray is travelling changes. The light ray appears to bend as it as it passes through the surface of the glass.


When a light ray passes from air into water a similar thing occurs: the light ray is bent as it strikes the surface of the water.

This 'bending of a ray of light' when it passes from one substance into another substance is called refraction.

The bending of a ray of light also occurs when the ray comes out of glass or water and passes into air.

The path of a refracted ray of light

What causes this bending?

Glass and water are thicker and heavier than air. They are said to be 'denser' than air.

What happens is that light slows down when it passes from the less dense air into the denser glass or water.

This slowing down of the ray of light also causes the ray of light to change direction. It is the change in the speed of the light that causes refraction.

This is a bit like when a car goes partly off the road and some of the wheels go into the softer edge of the road. The softer surface drags on the wheels and slows them down. This can cause the car to pull to the left, slightly changing its direction.

Direction of bending

When a ray of light passes from a less dense material (eg air) into a denser material (eg glass or water) it is bent away from the surface between the two materials. This means that in this situation the angle of refraction is always less than the angle of incidence.

Direction of bending

However, if the ray of light hits the surface at right angles (ie at 90°) to the surface, the ray is not bent.


When a ray of light passes from a denser material (eg water or glass) into a less dense material (eg air) it is bent towards the surface between the two materials. This means that in this situation the angle of refraction is always greater than the angle of incidence.

Direction of bending

You can apply these rules for the refraction of light to any prism, triangular or rectangular. They also apply to light passing through lenses.

Refraction with lenses

The bending of a ray of light also occurs when light passes into and out of a glass lens.

However, because of the curved surfaces of the lens, the bending causes the light rays in a beam either to come together (converge, picture on left-hand side) or to spread out (diverge, picture on right-hand side).

Converging rays Diverging rays

Because a convex lens can cause rays of light to converge, it can produce an image on a screen. This is called focusing an image onto a screen (a real image). The concave lens spreads the light rays out and so it cannot focus an image onto a screen.


© Commonwealth of Australia, 2003