THE NATURE OF LIGHT

Light is fundamental to photography. The

function of photographic materials is to record

patterns of light. The function of photographic

equipment used for taking pictures

is to produce light (lamps), to measure light

(exposure meters and color-temperature meters),

and to control light (lenses, shutters,

apertures, and filters). It is therefore desirable

for the photographer to possess some understanding

of the nature of light.

Light is defined as the form of radiant energy

that our eyes are sensitive to and depend

upon for the sensation of vision. The obvious

importance of light has resulted in its being

the object of an enormous amount of experimentation

and study over many centuries.

One of the first persons to make significant

headway in understanding the nature of light

was Isaac Newton. In the seventeenth century

he performed a series of experiments

and proposed that light is emitted from a

source in straight lines as a stream of particles.

This theory was called the corpuscular

theory.

However, the facts that light bends when it

passes from one medium to another, and

that light passing through a very small aperture

tends to spread out, are not easily explained

by the corpuscular theory. As a result,

Christian Huygens proposed another theory

called the wave theory. This theory holds

that light and similar forms of electromagnetic

radiation are transmitted as a waveform

in some media. (This theory was elaborated

considerably by Thomas Young in the nineteenth

century after he performed a number

of experiments.) The wave theory satisfactorily

explained many of the phenomena associated

with light that the corpuscular theory

did not, but it still did not explain all of them.

One of the more notable of these unexplained

effects was the behavior of black-body

radiation. (Blackbody radiation is radiation

produced by a body that absorbs all the radiation

that strikes it, and emits radiation by

incandescence, depending on its temperature.)

In 1900 Max Planck suggested the hypothesis

of the “quantization of energy” to

explain the behavior of blackbody radiation.

This theory states that the only possible energies

that can be possessed by a ray of light are

integral multiples of a quantum of energy.

In 1905, Einstein proposed a return to the

corpuscular theory of light with light consisting

of photons, each photon containing a

quantum of energy. These suggestions, along

with others, gradually developed into what

is known today as Quantum Theory or

Quantum Electrodynamics. This theory combines

aspects of the corpuscular and wave

theories, and satisfactorily explains all of the

known behavior of light. Unfortunately, this

theory is difficult to conceptualize, and can

be rigorously explained only by the use of sophisticated

mathematics. As a result, the corpuscular

and wave theories are still used to

some extent where simple explanations of

the behavior of light are required.