Man's physical universe

xanabras

434 EN ERGY MAY BE PROPAGATED BY VIBRATIONS

The success of signal buoys at sea and the red and white reflectors now

used so widely on bicycles, signs, guard rails, and house numbers

depends upon this principle.

There are two taillights on modern automobiles, which enable one to

judge how far ahead a car is at night.

If the lights appear to be close

together the car is far ahead, but

if they spread apart rapidly you

are approaching the car ahead at

a fast rate of speed.

Modern taillights

(reflex

have sections

reflectors) which will reflect

Fig. 202. Beam from a reflex reflector ^.^^ lig^^ from a car behind if the

type of taillight.

taillight bulb burns out.

The sky would appear black if it were not for the fact that the molecules

and dust of the atmosphere diff'use the light of the sun and moon

by reflecting a portion of the light. It can be shown that it is the dust

particles that are responsible for this difi^usion of the light of the sun

which causes the changing colors at sunrise and sunset, because, when

light is passed through a dust-free box, it is invisible although it can be

seen before it enters and after it leaves the box; light cannot be seen

unless it is coming toward one. Thus a powerful beam of light from a

searchlight could be passed through a dust-free room and leave it

complete darkness. The room would be illuminated at once, however,

by placing a white object in the path of the light. A useful application

of this idea is the placing of a white or light-colored object in front of an

automobile to reflect the light from the headlights when one is caught

with engine or tire trouble without a flashlight on a dark night.

The images of objects in flat mirrors differ from the original only in

that they are reversed. Mirror-writing is a common example of this

effect.

Invisible Glass Has Many Practical Uses.

C. Hawley Cartwright discovered that a film of magnesium fluoride

placed on glass makes it invisible by rendering it nearly perfectly

transparent. Ordinary glass reflects about 8 per cent of the incident

light and transmits about 92 per cent, while this new coated glass

transmits 99.6 per cent of the incident light. For the films to be rugged

enough to be practical in most cases, the transmission is increased to

only 98 per cent. Reducing the reflection 5 per cent for each surface,

however, reduces glare and ghosts 25 per cent.

This invention promises to have widespread application in eliminating

the glare from showcases, store windows, and glass-covered paintings.

in

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