Man's physical universe

xanabras

438 ENERGY MAY BE PROPAGATED BY VIBRATIONS

of window glass result in the light being refracted more at one point

than at another.

Light passing through homogeneous media having

parallel surfaces does not change direction.

The angle of refraction in a medium depends upon the angle of incidence,

and if the angle of incidence is great enough there will be no

Fig. 211. In this type of mirage two images are seen, an apparent one and

an actual one. The apparent image is due to the refraction of the light as it

enters an upper warmer layer of atmosphere.

refraction. This angle, which is the essential condition for total reflection,

is known as the critical angle.

On a hot day objects on the horizon seem to quiver and are not distinct

because the velocity of light in air varies with the temperature;

the light is therefore refracted as it passes through layers of air of different

temperatures and hence also of different densities.

Convection

currents of rising hot air become apparent because of the refraction

of the light as it passes from cooler to hotter portions of the air.

The twinkling of the stars is the result of the refraction of the starlight

as it passes through rising hot air currents or falling cold air

currents.

It has already been pointed out that telescopes are located

on mountain tops to avoid this effect as much as possible and that

the rooms containing telescopes cannot be heated for the same reason.

A type of mirage due to refraction is that produced by the bending

of light rays as they pass from chilled surface layers of the air into

warmer upper layers, as shown in

Fig. 212. A mirage. (From Optics

and Wheels, Courtesy of the General

Motors Corporation.)

Fig. 211. Such a mirage may be

formed over water with the result

that ships or land below the

true

horizon become visible.

Another type of mirage, illustrated

in Fig. 212, is the desert

mirage. In this case light from a

distant object is bent as it passes

from the cooler upper layers of air

to the hot surface layers of air until it reaches the critical angle X, at

which point the image is reflected as from a mirror. This same effect

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