You can't 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 the examples that can be seen by their own light. But most of the things you see are not giving off light of their own. They are just reflecting(反射) light that falls on them from the sun or some other luminous bodies. The moon, for example, doesn't give off any light of its own. It is not luminous. You see it because sunlight falls on it and some of the sunlight reflects on the earth. So moonlight is only second—hand sunlight.
    When you look at a book, it sends some of the light which fails on it to your eyes, and you see the book. If light could be kept out from where you're so that there would be no light from the book to reflect. Then you couldn't see the book even with your eyes wide open. Light travels so fast that the time in which it travels from the book you're reading to your eyes is as short as if there were no time at time. Light reaches us from the moon, which is about 380, 000 kilometres away, in only a little more than a second.