Summary

TM 9-258

the light quanta when emitted retained their identity as

medium or substance of constant optical density. Upon

individual packets of energy.

striking a different medium, light either will bounce back

(8) Millikan and Compton. Still later, R.A.

(be reflected) or it will enter the medium. Upon entering

a transparent medium, its speed will be slowed down if

Millikan's very accurate measurements proved that the

the medium is more dense or increased if the medium is

energy due to motion (called "kinetic energy) of

less dense. Some substances of medium density have

elemental units of light (called "photons") behaved as

abnormal optical properties and for this reason "optically

assumed by the Quantum Theory. Later proof was given

dense" would be a better term. If it strikes the medium at

by A.H. Compton in 1921 in determining the motion of a

an angle, its course will be bent (refracted ) upon

single electron and a photon before and after collision of

entering the medium.

Upon striking all material

these bodies. He found that both have kinetic energy

mediums, more or less of the light is absorbed.

and a momentum and that they behave like material

(1) Transmission of light energy. Visible light

bodies.

This was somewhat a return to the old

is one of many forms of radiant energy which is

corpuscular theory.

(9) Summary.

The present standpoint of

transmitted in waves. The means by which the waves

carry the energy can be illustrated in a simple manner.

physicists on the nature of light is that it appears to be

Secure one end of a rope to some object. Hold the other

dualistic, both particle and wavelike. It is assumed that

end of the rope, stretch it fairly taut, and shake it. A

light and electricity have much in common. It is presently

wave motion (wave pulse) will pass along the rope from

accepted that the energy associated with a light beam is

transmitted as small particle-like packets-photons,

the end that is held to the end that is secured (fig 2-1). If

the end of the rope is continually shaken, a series of

originally called quanta (para 2-1a (6) which may be

waves (wave train) will pass along the rope (fig. 2-2). It

described in terms of associated waves, but, which are

will be noted that the different parts of the rope (the

best measured relying solely on their particle-like nature.

medium) vibrate successively, each bending back and

The simple wave analogy is only a very rough

forth about its own position. The disturbance travels but

approximation.

However, the phenomena of light

the medium does not. Only the energy is carried along

propagation may be best explained in terms of wave

the rope. Now imagine that the light source is a vibrating

theory. The wave theory will be used in the remainder of

ball from which a countless number of threads extend in

this manual.

all directions. As the ball vibrates, successive waves are

b. Known Facts. All forms of light obey the same

transmitted out along the threads in all directions. In a

general laws. Light travels in waves in straight lines and

similar manner, light radiates from its source.

at a definite and constant speed, provided it travels in a

Figure 2-1. Wave carrying energy along rope.

2-2