Beth Wilson, age 12, of Fargo, N.D., for her question:
HOW DOES A MIRROR WORK?
A mirror is a smooth looking glass which reflects light rays rather than absorbs them. Most mirrors are made of a pane of glass which is coated on the back so that the light cannot pass through but is reflected. The amount of light reflected depends on the kind of material, the angle at which light strikes it and how polished the surface is.
When you stand close in front of a plane or flat mirror, your image is the same size as yourself. The image seems to be as far behind the mirror as you are in front of it. If you are three feet from the mirror, your image seems six feet away.
In a sense, the image in a mirror three feet in front of you is indeed six feet away because the light rays must travel six feet before you see them: Three feet to the mirror and then three feet back to your eyes.
The image in a mirror is always reversed. If you lift your right hand, your image seems to be raising its left hand.
If you want to see yourself in full length, your mirror must be at least half as tall as you are. This is because the angle of incidence must equal the angle of reflection.
Suppose the top of the mirror is about on a line with the top of your head. Then the light rays which come from your feet and which are reflected back to your eyes by the mirror must strike the mirror at least half the distance between your feet and head. If you are six feet tall, the mirror must be at least three feet high.
A concave spherical mirror is shaped like the inside of an imaginary sphere. This type of mirror is used as a reflector in automobile lights, bicycle lights and in mouth mirrors used by dentists.
Convex mirrors are just the opposite. They curve toward the object instead of inward, away from it. Some auto rear view mirrors are convex.
Both convex and concave mirrors are used for amusement in some county fairs and carnivals. People stand before a convex mirror and see themselves stretched out grotesquely. Then they look in a concave mirror and see themselves short and round.
The more mirrors are polished, the more light they will be able to reflect. But even the best mirrors never reflect all the light which falls upon them.
The angle at which light strikes the mirror from the object is called the angle of incidence. The angle at which light is reflected is called the angle of reflection.
The angle of incidence is always equal to the angle of reflection. A perpendicular to the mirror which strikes the mirror at the point of reflection is called the normal.
Reflection is the return of a wave of energy after it strikes a surface.
The principle of reflection has many applications in daily living. The mirror, or a piece of glass coated with silver, reflects most of the light striking it. Polished surfaces, such as chromium, also reflect most of the light that strikes them. Clear surfaces, such as window glass, reflect little light.