Waves can travel through many different substances. For example, light waves can travel through air, water, glass and many other objects. These substances are said to be transparent to those waves.

When a wave moves from one substance to another the wave changes direction: or is refracted.

Look at the photograph of the glass of water. Light waves are travelling from the stick; through the water; through the glass; then through the air until they hit the camera lens.

By travelling through water, glass and air, the light waves are bent slightly (or refracted).
This makes the stick look as though the bit under the water is broken.

Light waves can even be refracted when they move from hot air into colder air. On very hot days the air above a tarmac road seems to shimmer. This is the hot and cold air refracting the light waves above the hot tarmac.
The same effect causes people to see mirages in hot deserts: people think an oasis is right in front of them when really it is many miles away - the hot air has refracted light making the oasis seem closer.

Light gets refracted as it passes through a piece of glass. You can see from the picture that the light gets refracted twice: once when it changes from air to glass; and again when it changes from glass back to air.

If you change the shape of the glass you can control the way in which the light bends. This is what a lens does.

In a lens the light gets refracted twice: once when it leaves the air and travels through the glass; and again when it leaves the glass and travels back into the air.