ELECTROMAGNETIC THEORY OF LIGHT
James Clark Maxwell, a brilliant Scottish scientist Of the middle l9th century, showed, by
constructing an oscillating electrical circuit, that electromagnetic waves could move through empty space.
Light eventually was proved to be electromagnetic.
Current light theory says that light is made up of very small packets of electromagnetic energy called
PHOTONS (the smallest unit of radiant energy). These photons move at a constant speed in the medium
through which they travel. Photons move at a faster speed through a vacuum than they do in the
atmosphere, and at a slower speed through water than air.
The electromagnetic energy of light is a form of electromagnetic radiation. Light and similar forms
of radiation are made up of moving electric and magnetic forces and move as waves. Electromagnetic
waves move in a manner similar to the waves produced by the pebble dropped in the pool of water
discussed earlier in this chapter. The transverse waves of light from a light source spread out in expanding
circles much like the waves in the pool. However, the waves in the pool are very slow and clumsy in
comparison with light, which travels approximately 186,000 miles per second.
Light radiates from its source in all directions until absorbed or diverted by some substance (fig.
1-17). The lines drawn from the light source (a light bulb in this instance) to any point on one of these
waves indicate the direction in which the waves are moving. These lines, called radii of the spheres, are
formed by the waves and are called LIGHT RAYS.
Figure 1-17.Waves and radii from a nearby light source.
Although single rays of light do not exist, light "rays" as used in illustrations are a convenient
method used to show the direction in which light is traveling at any point.
A large volume of light is called a beam; a narrow beam is called a pencil; and the smallest portion
of a pencil is called a light ray. A ray of light, can be illustrated as a straight line. This straight line drawn
from a light source will represent an infinite number of rays radiating in all directions from the source.
Q31. What are three means of producing light?
Q32. What is the smallest unit of radiant energy?