simple lenses

Let's make some simple lenses and see how they bend light.

Caution: This activity uses hot water and a knife. Have an adult assist you with this activity.

What you need

To do this activity you will need:

·         a torch

·         a comb

·         two packets of jelly crystals, both the same flavour and preferably a light colour or clear. In some parts of the world, jelly is called jello.

·         a rectangular plastic container, such as a lunch box, to use as a jelly mould

·         hot water

·         a spoon

·         a fridge

·         a dark room

·         a knife with a straight edge

·         a cutting board

·         a table

·         an adult.

What to do

1. Mix up your jelly, but only use half as much water as normal. You need to make very stiff jelly.
2. Pour the jelly into your mould and leave it to set overnight.
3. When the jelly has set, tip it out onto the cutting board. It is easier to tip out if you run a knife around the edge and then dip the outside of the mould into warm water for about ten seconds.
4. Use the knife to cut the jelly into the shape of some lenses such as:
   ·         wide in the middle and thin at the ends
   ·         thin in the middle and wide at the ends
   ·         semicircular.
5. Shine light from the torch through the lenses and look at how they bend the light. If you place a comb between the torch and the lenses, you will get lines of light, which can make it easier to see the light bending.
6. After you have finished, you can eat the jelly.

What's happening

When light hits a boundary between two substances, like the surface of the jelly, it often bends. This is called refraction. When light goes from air into the jelly, it bends away from the surface of the jelly. When it goes from jelly into the air, it bends towards the surface of the jelly. When the jelly surface is curved, rays of light hitting at different spots on the surface will either spread out or move together.

If the jelly makes a concave lens (thick on the ends, thin in the middle) the rays of light will be spread out.

If the jelly makes a convex lens (thin on the ends, thick in the middle) the rays of light will bend towards each other, until they cross over and then spread out again. If you are lucky and your lens is just the right shape, you may even find the rays of light all cross over at one spot.

The strength of a lens depends on its shape and the material it is made from. Different materials will bend light by different amounts.