Put your head on a green chinrest at the long table and fold away all the lenses. Can you see the face on the other side of the table? It is now reflecting the light. These reflected rays of light fall on your eyes and therefore you can see it. Exactly as it is. Normally, the light rays go straight ahead. But lenses change this; they can change the direction of the light rays.
This table only has convex lenses. A convex lens is thicker in the centre than at the edges. Light rays passing through such a lens go towards each other. They intersect at one point. This is called the “focal point”. The more convex the lens, the closer the focal point is to the lens. A convex lens can enlarge, reduce and turn images upside down.
There are also concave lenses. These are thicker at the edges than at the centre. Light rays passing through a concave lens move away from each other. A concave lens can make images smaller.
Lenses in your eyes
You use lenses every day, even as we speak! Our eyes contain a convex lens. If your eyes are healthy, the focal point falls exactly on the retina when you look at something. That is why you see sharply.
Something strange is going on, however. You can look at something that is close by as well as something that is far away… and yet you can always see it clearly! This is because our eye lens is quite special. It can flatten or curve itself. So it adjusts where the focal point falls.
If you are near-sighted or farsighted, your lens no longer adjusts properly. Then your eyes need some help from glasses or contact lenses.
