On a hot day, how is it that you see "heat waves" rising from the street? What makes the stars twinkle? Light usually travels in straight lines. If the light from an object comes from straight in front of you, you know that the object is straight in front of you. But you can bend light so that it seems to come from a different place, thus making things seem to be where they are not. EXPERIMENT 44. Hold a triangular glass prism vertically (straight up and down) in front of one eye, closing the other eye. Look through the prism, turning it or your head around until you see a chair through it. Watch only the chair through the prism. When you are sure you know just where it is, try to sit down in it. Now look for a pencil or a piece of chalk through the prism, in the same way. When you think you know where it is, try to pick it up. The reason the chalk and chair seem to be where they are not is that the prism bends the light that comes from them and makes the light seem to come from somewhere else. As you already know, when you look at a chair you see the light that reflects from it. You judge where the chair is by the direction from which the light is coming when it reaches your eye. But if the light is bent on its way, so that it comes to your eye as it ordinarily comes from an object off to one side, naturally you think the thing you are looking at is off to one side. Maybe the diagram (Fig. 65) will make this clearer. [Illustration: FIG. 65. In passing through the prism the light is bent so that an object at _b_ appears to be at _c_.] Here in _a_ is an object the same height as the eye. The light comes straight to the eye, and one knows that the object is level with the eye. In _b_ the object is in the same position as in _a_, but the prism bends the light so that it strikes the eye with an upward slant. So the person thinks the object is below the eye at _c_. Here is another experiment with bending light: EXPERIMENT 45. Fill a china cup with water. Put a pencil in it, letting the pencil rest at a slant from left to right. Lower your head until it is almost level with the surface of the water. How does the pencil look? [Illustration: FIG. 66. The pencil is not bent, but the light that comes from it is.] The reason the pencil looks bent is because the light from the part of it under the water is bent when it passes from the water into the air on its way to your eye; so the slant at which it comes to your eye is the same slant at which it ordinarily would come from a bent pencil. EXPERIMENT 46. Fill a glass with water. Put the pencil into it in the same way you put it in the cup in the previous experiment, letting the pencil slant from left to right. Lower your head this time until it is on a level with the water in the glass, and look through the glass and water at the pencil. Notice what happens where the pencil goes into the water.