The Electromagnetic Spectrum

Abstract

Light travels from the sun to the earth, a distance of about 145 000 000 km, through space containing very little material. When absorbed by a surface it is converted into heat, a form of energy. Energy must therefore have arrived from the sun across this immense distance. In fact almost all the energy known to man has been derived from the sun either now or in past ages. Thus the sun’s energy, which millions of years ago was responsible for the growth of luxurious vegetation, is now available to us in the form of coal. To account for this transfer of energy over such a large distance we must know something of the nature of light. Energy can pass from one place to another in two ways. The kinetic energy of a moving body which obeys the laws of mechanics is the essential feature of the corpuscular theory as advocated by Newton at the close of the seventeenth century on the basis of the experimental evidence known to him at that time. On the other hand, energy can also pass from one place to another by a wave motion. This was the basis of the wave theory of light supported by Hooke and Huygens. Sound was then known to be a wave motion and the fact that one could hear but was unable to see around corners proved to be a serious obstacle to the acceptance of the wave theory of light for over a hundred years even though it was known that light deviated very slightly from its straight line path on passing close to the edge of an obstacle.