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 that can be ssen 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 bodies. Then 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 secondhand sunlight. When you look at a book, it sends your eyes some of the light that 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 it took no time at all. Light reaches us form the moon , which is about 380,000 kilometers away, in only a little more than a second. 小题1:You can see the book because___________.
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