When a wave or light ray moves from one medium to another its speed changes. The direction of the ray may also change.

This property of waves is called refraction and commonly occurs with light rays.

A good example of refraction is when you see the bottom of a swimming pool. The light travels from the bottom of the pool, through the water, then through the air into your eye. The light travels in such a way that the pool often appears to be shallower than it really is.

A boy looks into a pond containing a fish. Arrows show that the light from the fish to the boy's eyes leaves the water at an angle, so the fish is actually lower in the water than it appears. The apparent depth and actual depth are labelled.

A normal is a dotted line drawn perpendicular to the surface of the refracting material, at the point of entry of the light.

When light travels from air into a denser medium like water or glass, it will refract towards the normal.

When light travels from a denser medium into air, it will refract away from the normal.

Example: Light rays passing through a glass block

Step 1

First of three images: A light wave enters a block of glass perpendicular to the surface, then travels through and out of it in a straight line without bending. Labels indicate that the wave slows and its wavelength decreases as it enters the glass, and it returns to its original values as it returns to the air.

Step 2

2/3: light wave enters glass block at angle to surface, changes direction as it travels through and is bent to original direction as it leaves. Speed & direction change as ray enters the block.

Step 3

3/3: wave enters block at angle to surface. Angle to normal labelled 'angle of incidence' when entering & 'angle of refraction' as bent inside block. Angle of refraction less than angle of incidence.

As can be seen in the diagram the light ray changes direction as it enters and leaves the block.

In all ray diagrams, all angles of incidence and refraction are measured between the ray and the normal.