Why do we have in a camera a lens(镜头)instead of a simple hole?
The reason can be seen from the figures(图像).
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.
1. In Figure 1 we see a faint small point on the wall because .
A. the point(P1) is very small B. the hole isn’t big enough
C. light rays don’t travel in straight lines D. light rays can’t pass through a small hole
2. Figure 2 shows that the bigger the hole is, .
A. the more light can pass through B. the clearer the picture will be
C. the better result we will get D. the faster the light rays travel
3. From figure 3, we can see a lens .
A. can form a clear picture B. can make light go in a straight line
C. can help light rays to go faster D. cannot give the picture more light than in Fig. 2
4. The main idea of the second paragraph of the article is that .
A. a smaller hole is better than a bigger one
B. big holes are better than small ones
C. both a big hole and a small one have their weak points
D. light rays are sure to pass through a hole no matter it is big or small
5. Which of the following statements is true?
A. Light can go through all kinds of materials
B. A camera can’t be made without a lens
C. The lens is only used in a camera
D. Most of the light we get is from the lens
答案：
1B 2 A 3 A 4 D 5 B
