How can light get through a glass window and I can’t?
Light is different from you and I. For one thing, we have mass. That means here on earth, where we have gravity, we tend to stay on the ground. That’s good for us – we don’t go floating around. But light has no mass. That’s good for us too. Could you imagine if the light from your TV set fell to the ground before it got to us? That would be no fun at all.
So if light doesn’t have mass, what does light have? Energy! It’s this energy that lets our eyes see light, lets our bodies be warmed by the sun, and gives plants the ability to change carbon dioxide and water into plant stuff.
That means that light is energy and it travels along at, you guessed it, the speed of light. The speed of light depends on how the energy in the light reacts with the material it is traveling through.
Then what happens when it gets to a clear glass window? When I get to a window, my mass interacts with the mass of window. And, that keeps me from going through it. But light has no mass. When it gets to a window its energy interacts with the material in glass just as it did when it was in air. It travels through the glass. But as glass has different physical properties than air there is a slight difference. It travels a bit more slowly in glass. We can use this effect to bend light rays as we do with eyeglasses and telescopes.
While light can go through clear glass, it can’t go through everything. When light hits an object, some of it goes into the object, and some is reflected. In the case of a mirror, most of it is reflected. How about the light that goes into the object? We have a few choices there as well. In the case of sunglasses, some of the light energy is absorbed and converted to heat, and the rest goes through the lens giving us a more comfortable view on a sunny day.
Sometimes when light is absorbed by an object, it is converted into another form of energy. In the case of a tree, sunlight is converted to chemical energy by the tree. When you burn a log in a fire place, you’re releasing energy that started out as sunlight. Another way to convert that absorbed energy into another form is with photo cells that you might see on some roofs. Photo cells have the ability to convert light energy directly into electricity.
So when two objects with mass interact, they interact physically. We bump into thing. When light interacts with an object, it’s a matter of energy. And, that lets light shine through a glass window.
