Light: Mirrors and Lenses
Converging and diverging lenses
Test what different lenses do to a beam of light.
Do lenses bring beams together or spread them out, like curved mirrors? Activity 10.10 tests three things at once. You need a thin glass plate, a convex lens, a concave lens, a torch and comb (for several parallel beams), a paper clip, two identical books and white paper.
Put the two books side by side and stand the glass plate or a lens upright between them, with white paper on both books so you can see the beams going in and coming out. Shine the parallel beams through each in turn.
| What the beams pass through | What happens |
|---|---|
| thin flat glass plate | they pass through unchanged, still parallel |
| convex lens | they come together: it converges the light |
| concave lens | they spread apart: it diverges the light |
So a convex lens is also called a converging lens, and a concave lens a diverging lens.
Activity 10.11 repeats the burning-paper test with a convex lens in the sunlight instead of a concave mirror, and yes, the paper burns. Never look at the Sun, directly or through the lens: it can damage your eyes.
Lenses are everywhere: in spectacles, cameras, telescopes and microscopes. And your own eye has a convex lens inside it, an amazing one that can change its shape, so you can read a book and then look at something far away.
More than 800 years ago, in the time of the mathematician Bhāskara II, Indian astronomers used shallow bowls of water to watch stars and planets, reading their positions from reflections seen through tubes set at the right angles. The laws of reflection aren't written down in those books, but the instruments suggest they understood them in practice.
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