How We See> Stars are many, many millions of miles away. Yet we can see them, lots of them. Why? Like our sun, far-off stars radiate light. That is, they send light in all directions through space. A tiny bit of this light travels the millions of miles to Earth. An even tinier bit shines directly to our eyeballs. This tiny amount of light hitting our eyes allows us to “see” the star. If the light didn’t make the whole trip from star, to Earth, to your eyes, you could never see the star.
A burning candle, glowing lava, and a light bulb are other direct sources of light. But sometimes, light rays reach our eyes indirectly. In a dark room, shine a flashlight at a wall. Can you see the beam of light between the flashlight and the wall? It depends on how dusty your room is. When light hits dust, you can see the beam. It becomes visible. Without dust, the beam is invisible. Light is there. But we can’t see it.
We can only see light when it shines in our eyes. It can shine directly in our eyes. Or, it can reflect (bounce) off something, such as bits of dust in the air.
To find out where light goes when it travels, we trace the path the light takes. Because light travels in straight lines, it’s easy to imagine a bunch of arrows heading off in all directions from a source of light. Each of these arrows is what we call a “ray” of light, and it travels in a straight line between bounces. We can tell where the light goes by tracing these rays of light until they reach our eye.
