Why do we have in a camera a lensinstead of a simple hole? The reason can be seen from the figures. In Figure 1, the hole is small. Rays of light from a point (P1)outside reach a very small part of the wall opposite, and we see there a small point. But when the hole is bigger, as in Figure 2, rays from the point(P2)can cover a larger part of the wall opposite, and we don’t see a clear point, Rays from other points(Q) outside can also fall on the same place inside. Therefore the picture is not clear when the hole is big and it is not bright when the hole is small because very very little light can pass through it.. We can get better result with a lens. If the lens is made in the shape shown in Figure 3, all the rays of light from the point(P3) are thrown on point(P’) inside. The picture which we see, therefore, is clear, and it is also bright because more light can pass through a lens than through a small hole. 【查看更多】
