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?
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.