Interaction Between
Light and Matter
Light may be considered as either a wave
motion or as a stream of particles, photons, depending on which is the most
practical physical explanation of a particular optical effect.
When considering light as a wave, the important characteristics by which it can
be defined are quantities such as wavelength, wave number and frequency.
Visible and ultraviolet light have wavelengths in the nanometer range i.e.
1/1000000000 m. The wave number is the reciprocal of the frequency, or the
number of complete transmitted wave cycles which lie within a stated distance,
usually one centimeter. The frequency can be stated as the number of complete
cycles which pass a fixed point during one second. The relationship can be
expressed by the formula v=c/lamda where v=frequency, c=the speed of light and
lamda=the wavelength.
Even studying the interchange between light and matter it is most practical to
use the photon theory. These particles have a certain energy, which they
can release if they collide with an atom, a molecule or a photoelectric cell.
The energy contained in a photon, E, is determined by the frequency of the
light in conformation with the formula
E= h x v
where h is Planck's constant. Hence the higher the frequency the more
energy is contained in each photon. In contradiction to what we normally
mean by the word "particle", photons have no mass when at rest; the
mass they obtain is only due to their movement, in accordance with Einstein's
famous formula
E = m x c x c