CHAPTER 2 PROPERTIES OF LIGHT

TM 9-258

CHAPTER 2

PROPERTIES OF LIGHT

Section I. LIGHT

Fresnel (1788-1827), and others supported the wave

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Theories and Known Facts About Light.

theory and the rival corpuscular theory was virtually

a. General. The true nature of light and the

abandoned. These scientists offered their measurement

manner in which it travels have fascinated scientific

of light waves as proof. They accepted the "ether" theory

investigators for centuries. Many theories have been

and assumed light to be waves of energy transmitted by

advanced to explain why light behaves as it does. Later

an elastic medium ether.

discoveries have proved some of these theories

(5) Maxwell, Boltzmann, and Hertz. Light and

unsound. This manual cannot attempt to cover all the

electricity were similar in radiation and speed was proved

aspects of light nor is this knowledge essential to the

the experiments of Maxwell (18311879), Boltzmann

study of the laws of optics as they apply to fire control

(1844-1906), and Hertz (18571894).

From these

equipment. Therefore, only a short summary of the

experiments was developed the Electromagnetic Theory.

various theories of light is given.

These experiments produced alternating electric currents

(1) Ancient Greek theory. The earliest known

of short wavelengths that were unquestionably of

speculations as to the nature of light were those of the

electromagnetic origin and had all the properties of light

ancient Greeks. They believed that light was generated

waves. The Maxwell theory contended that energy was

by streams of particles which were ejected from the

given off continuously by the radiating body.

human eye. This theory could not persist because it

(6) Planck. For a time it was thought that the

could not explain why one could not see as well at night

puzzle of light had been definitely solved but, in 1900,

as by day.

Max Planck refuted the Maxwell theory that the energy

(2) Newton. The first modern theory was that

radiated by an ideal radiating body was given off

of Sir Isaac Newton (1643-1727). This, the corpuscular

continuously. He contended that the radiating body

theory, assumed light to be a flight of material particles

contained a large number of tiny oscillators, possibly due

originating at the light source. Newton believed that light

to the electrical action of the atoms of the body. The

rays moved at tremendous velocity in a state of near

energy radiated could be high frequency (para 2-3b) and

vibration and could pass through space, air, and

with high energy value, all possible frequencies being

transparent objects. This theory agreed with the property

represented. The higher the temperature of the radiating

of light to move only in a straight line, in a medium of

body the shorter is the wave length (para 2-3a) of most

constant optical density, but failed to explain other

energetic radiation. In order to account for the manner in

phases of light behavior. Newton stumbled on light

which the radiation from a warm black body is distributed

interference in his discovery of Newton's Rings (para 2-

among the different wave lengths, Planck found an

44)but did not realize their significance.

equation to fit the experimental curves, and then only on

(3) Huygens. In Newton's time, Christian

the very novel assumption that energy is radiated in very

Huygens (1629-1695) attempted to show that the laws of

small particles which, while invisible, were grains of

reflection and refraction of light could be explained by his

energy just as much as grains of sand. He called these

theory of wave motion of light. While this theory seemed

units quanta and his theory the Quantum Theory. The

the logical explanation for some phases of light behavior,

elementary unit or quanta for any given wave length is

it was not accepted for many years because a means

equal to hn, where n is frequency of the emitted radiation

was lacking by which to transmit the waves. Huygens

and h is a constant known as Planck's constant. Quanta

proposed that a medium, which he called "ether, " be

set free were assumed to move from the source in

accepted as existing to serve light rays as water does the

waves.

familiar waves of water. He assumed this medium to

(7) Einstein. A few years later, Albert Einstein

occupy all space, even that already occupied by matter.

backed up in Planck and contended that

(4) Young and Fresnel. Experiments made

about 50 years later by Thomas Young (1773-1829),

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