A mirror that magnifies. A convex lens is not the only thing that can magnify. A concave mirror, which is one that is hollowed out toward the middle, does the same thing. When light is reflected by such a mirror, it acts exactly as if it had gone through a convex lens (Fig. 85).
Experiment 49. Place the lighted candle and the paper about 4 feet apart, as you did in Experiment 47. Hold a concave mirror back of the candle (so that the candle is between the mirror and the paper); then move the mirror back, the mirror casting the reflection of the candle light on the paper, until a clear image of the candle is formed.
Look at your image in the concave mirror. Does it look larger or smaller than you?
How telescopes are made. Astronomers use convex lenses in some of their telescopes; in others, called reflecting telescopes, they use concave mirrors. Both do the same work, making the moon, the planets, and the sun look much larger than they otherwise would.
Application 37. Explain how a reading glass makes print look larger; how you can see germs through a microscope; what kind of mirror will magnify; what kind of lens will magnify.
Inference Exercise
Explain the following:
221. The water that forms rain comes from the ocean, yet the rain is not salty.
222. Iron glows when it is very hot.
223. You can start a fire with sunlight by holding a reading glass at the right distance above the fuel.
224. Big telescopes make it possible for us to see in detail the surface structure of the moon.
225. A room is lighter if it has white walls than if it has dark walls.
226. Iron is heated by a blacksmith before he shapes it.
227. A dentist's mirror is concave; he sees your teeth enlarged in it.
228. Good penholders usually have cork or rubber tips.
229. A man's suit becomes shiny when it gets old.
230. When you look at a window from the sidewalk, you frequently see images of the houses across the street.
Section 26. Scattering of light: Diffusion.
Why is it that on a dark day the sun cannot be seen through light clouds?
Why do not the stars come out in the daytime?
If you were on the moon, you could see the stars in the daytime. The sun would be shining even more brightly than it does here, but the sky around the sun would be pitch black, except for the stars shining out of its blackness. The reason is that there is no air on the moon to scatter the light.
Why we cannot see the stars in the daytime. Most of the sun's light that comes to the earth reaches us rather directly; that is why we can see the image of the sun. But part of the sunlight is scattered by particles of air, and that is why the whole sky is bright in the daytime. You know, of course, that the blue sky is only the air that surrounds the earth. Enough of the light is scattered around to make the sky as bright as the stars look from here; so we cannot see the stars through the sky in the daytime.