Suppose you are reading a book. Suppose, too, you suddenly close your eyes. Can you still see the book? "Of course not," you will say. But can you tell why? You would probably say, "When I close my eyes my eyesight cannot get out of my eyes to get to the book. " But this explanation is wrong.      You cannot see any object unless light from that object gets into your eyes. Some of the things you see give off light of their own. The sun, the stars, a lighted lamp are examples of that can be seen by their own light. Such things are luminous. Most of the things you see are not giving off light of their own. They are simply reflecting   light that falls on them from the sun or some other luminous body. The moon, for example, does not give off any light of its own. It is non-luminous. You see it because sunlight falls on it and some of it reflects in your direction. So moonlight is only second-hand sunlight.     When you look at a book, it sends to your eyes some of the light which falls on it, and you see the book. If light could be kept out from where you are so that there would be no light for the book to reflect, then you could not see the book even with your eyes wide open.      Light travels so fast that the time in which it travels from the book you are reading to your eyes is so short as if there were no time at all. Light reaches us from the moon, which is about 380,000 kilometres away, in only a little more than a second. _   have light of their own.
The sun and the moon The stars and the earth The sun and the stars The moon and the earth