Application 34. Some boys were playing war and were in a ditch that they called a trench. They wanted to make a simple periscope so that they could look out of the ditch at the "enemy" without being in danger. They had an old stovepipe and a mirror. Practically all of them agreed that if the mirror were fixed in the top of the stovepipe and if they looked up through the bottom, they would be able to see over the side of the ditch. But they had an argument as to how the mirror should be placed. Each drew a diagram to show how he thought the mirror should be arranged, using dotted lines to show how the light would come from the enemy to their eyes. Three of the diagrams are shown in Figure 64.
The boy who drew the first said: "If you want to see the enemy, the mirror's got to face him. Then it will reflect the light down to your eyes."
The boy who drew the second said: "No, the light would just go back to him again. The mirror must slant so that the light that strikes it at a slant will be reflected to your eye at the same slant."
"How could it get to your eye at all," the third boy said, "if the mirror didn't face you? You've got to have the mirror reflect right down toward your face. Then all the light that strikes it will come down to you."
Which arrangement would work?
Inference Exercise
Explain the following:
191. Your hands do not get wet when you put them into mercury.
192. When beating hot candy, we sometimes put it in a pan of water.
193. Electric stoves frequently have bright reflectors.
194. We put ice in the top of a refrigerator.
195. You can jack up the back part of an automobile when you could not possibly lift it up.
196. The sun shines up into your face and sunburns you when you are on the water.
197. People in the tropics dress largely in white.
198. Menthol rubbed into your skin makes it feel very cold afterward.
199. We feel the heat of the sun almost as soon as the sun rises.
200. You can shoot a stone far and hard with a sling shot.
Section 23. The bending of light: Refraction.
How do glasses help your eyes?
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.