|MadSci Network: Physics|
Why don't you shadows in a mirror? Consider the following situation: a light bulb in the center of a room casts light. You place a tennis ball between the light bulb and a mirror on the wall. The tennis ball casts a shadow on the mirror. Now, how are you going to look at the shadow? Well, if you stand off to the side a bit (say, to the right), so that you don't interfere with the tennis ball's shadow, then when you look at the mirror, you are seeing light rays which have come from the _other_ side of the tennis ball (from the left), bounced off the mirror, and reached your eyes. The light rays "in the shadow" travel straight from the tennis ball to the mirror and bounce straight back to the tennis ball. Because you are standing off to the side, they don't reach your eyes. So, suppose you stand directly behind the tennis ball, so that those light rays "in the shadow" will bounce straight off the mirror to you. Ooops -- now YOU are blocking all the light from the bulb which passes close to the tennis ball. If the light bulb is the only source in an otherwise dark room, then you will see a silhouette of your head in the mirror. You won't be able to see your eyes and nose very well, because they are in the dark. In effect, you are seeing a shadow (of yourself) in the mirror. In order to see clearly the shadow of some other object in a mirror, you can try the following: go into a dark room with a single light source. Stand off to the side of the light source, and place a mirror at an angle so that it reflects the light from the source to you (i.e. so that you see the light source in the mirror. Now, have someone else hold a piece of cardboard between the mirror and the light source. The cardboard will block the light rays from reaching the mirror -- casting a shadow on it. When you look at the mirror, it will be much darker than before -- you are seeing the shadow of the cardboard. In ordinary circumstances, we are used to seeing shadows as diffuse dark patches on surfaces; people standing just about anywhere will be able to see the dark patches and agree on their appearance. This is a consequence of the nature of most surfaces (like walls and floors): they are rough, with little pits and grooves and hills that reflect light in all directions. Light falling on such a surface is reflected in all directions, so people everywhere agree that the surface is bright. When light is blocked so that a shadow is cast on the surface, everyone can see that the light from the wall is diminished, and they all say, "There's a shadow." But a mirror is a very smooth surface, which reflects light only in one direction. Different people in different positions will NOT agree on the brightness of a mirror, and they will NOT detect the presence of a shadow equally. You also ask about very dark surfaces: would shadows be invisible on them? Well, imagine a wall made of a material which reflects no light at all. Anyone who looks at the wall will see -- nothing. No light. It will be perfectly black. Shine a flashlight on the wall -- and you see nothing. Now block the flashlight's beam with a piece of cardboard. Look at the wall. What do you see? Nothing. There's no difference, whether you block the beam or not. So, you don't notice a shadow. No actual surface reflects absolutely zero light, so no actual surface will do this. But it is difficult to detect shadows cast on dark surfaces.
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