Man's physical universe

xanabras

LIGHT MAY BE REFLECTED AND REFRACTED 435

It has already been applied in making camera lenses "faster." This

"speeding-up" of lenses is especially valuable in color photography

and television.

The thickness of the film is four

millionths of an inch, and it

must

be carefully controlled because the

secret of the success of the film depends

upon its thickness. A film of

this thickness is such that the light

rays reflected by the two surfaces

of the film cancel each other by

interference.

Concave and Convex Mirrors Are

Used Like Lenses to Focus

or Diverge Light.

Mirrors of curved shape are used

as reflectors in automobile headlights.

Light from a distant object

will be brought to a focus at the

point where the lamp is located.

This is the principle of the reflecting

telescope, flashlights, and

parabolic reflectors of automobile

headlights. On the other hand, an

automobile lamp placed at this

point of focus will reflect long

Fig. 203. This reflection on the

face of C. Hawley Cartwright shows

how a thin film decreases the light

reflection from glass. Sunlight is reflected

from the glass to his cheek.

The round spot in the center of the

piece of glass was coated with a thin

film of magnesium fluoride. (Courtesy

of Science Service.)

parallel beams.

Concave mirrors can produce three kinds of images depending upon

the position of the object: (1) if the object is very close to the mirror,

the image will be upright but larger than the object — dentists' exploratory

mirrors use this principle; (2) if the object is placed somewhat

farther from the mirror, the image will be smaller than the object and

Fig. 204.

In the automobile headlight the lamp is placed at the focal point of the

concave mirror.

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