听力原文： Sight is not something that reaches out from our eyes. Instead it is the light that travels to our eyes. You see this page, for example, because light, reflecting from the sun or an electric light, travels from the paper to your eyes. Sometimes we see light as it comes from a direct source, such as the sun, fire, lighting, or a light bulb. The rest of the time we see light as it is reflected off objects. It must have been a great leap in the intuition of scientists to realize that light actually travels. It isn’t just there! In the air light travels at a speed of 186,000 miles per second. It travels slightly faster in a vacuum and slower in other trans- parent materials such as water or diamonds. It takes light less than one minute to travel from the earth to the moon and about 15 minutes to go from the earth to the sun. (1)