5 The Incredible Quantum Mechanics
79
Physics had by now been discovered, with only a few details remaining
to be clarified. The ground for such optimism was the recently proposed
theory of electromagnetic fields and waves by James Clerk Maxwell. In his
seminal papers and book [1], Maxwell provided a convincing explanation of
all known electric and magnetic phenomena. His theory, which also gathered up earlier work by Faraday and Gauss, predicted many as yet unknown
effects, 2 which were subsequently discovered. In addition, Maxwell succeeded
in unifying two forces (electric and magnetic), which up to then had always
been considered distinct and independent. Maxwell’s achievement has been
called “the second grand unification” , the first one being Newton’s unification
of terrestrial and celestial mechanics.
But perhaps the greatest merit of Maxwell’s work was to show that light
(or, more precisely, the propagation of optical waves) was nothing but the
solution of his equations in the most elementary case of a completely empty
medium (vacuum). This was remarkable, since all other cases of propagating
waves require a medium of some substance to support them, e.g. air for
acoustic waves (sound) and water for sea waves. Luckily, light can also propagate, although at the cost of some absorption, in tenuous media such as air.
This applies not only to visible light (i.e. radiation in the range of frequencies
which can be captured by the human eye). It also applies to x-rays, gamma
rays, ultraviolet and infrared light, as well as to waves used for the propagation of radio and television channels. All these forms of radiation propagate
in exactly the same manner, as predicted by Maxwell’s equations, and are
distinguished only by their different frequencies.
Frequency, along with wavelength and velocity, are the three quantities that
characterise all forms of wave motion. When we throw a pebble into a pond,
we can observe these properties readily. By concentrating our attention on
one point in the pond, we can see the water surface rising and falling as the
wave moves by. The number of times the surface rises and falls per second
at this point is the frequency of the wave. On the other hand, if we concentrate on the wavefront itself, and measure how far the wave progresses in one
second, we have the velocity of the wave. The third quantity, the wavelength,
is the distance between successive wavefronts. A little thought will convince
the reader that these three quantities are not independent; in fact, the velocity
is equal to the product of the other two.
For some types of waves, the velocity is different for different frequencies.
An example is ocean swell. These long wavelength waves are generated by
2 For example, in 1887 Heinrich Hertz discovered microwave electromagnetic radiation, as predicted
earlier by Maxwell.
79
Physics had by now been discovered, with only a few details remaining
to be clarified. The ground for such optimism was the recently proposed
theory of electromagnetic fields and waves by James Clerk Maxwell. In his
seminal papers and book [1], Maxwell provided a convincing explanation of
all known electric and magnetic phenomena. His theory, which also gathered up earlier work by Faraday and Gauss, predicted many as yet unknown
effects, 2 which were subsequently discovered. In addition, Maxwell succeeded
in unifying two forces (electric and magnetic), which up to then had always
been considered distinct and independent. Maxwell’s achievement has been
called “the second grand unification” , the first one being Newton’s unification
of terrestrial and celestial mechanics.
But perhaps the greatest merit of Maxwell’s work was to show that light
(or, more precisely, the propagation of optical waves) was nothing but the
solution of his equations in the most elementary case of a completely empty
medium (vacuum). This was remarkable, since all other cases of propagating
waves require a medium of some substance to support them, e.g. air for
acoustic waves (sound) and water for sea waves. Luckily, light can also propagate, although at the cost of some absorption, in tenuous media such as air.
This applies not only to visible light (i.e. radiation in the range of frequencies
which can be captured by the human eye). It also applies to x-rays, gamma
rays, ultraviolet and infrared light, as well as to waves used for the propagation of radio and television channels. All these forms of radiation propagate
in exactly the same manner, as predicted by Maxwell’s equations, and are
distinguished only by their different frequencies.
Frequency, along with wavelength and velocity, are the three quantities that
characterise all forms of wave motion. When we throw a pebble into a pond,
we can observe these properties readily. By concentrating our attention on
one point in the pond, we can see the water surface rising and falling as the
wave moves by. The number of times the surface rises and falls per second
at this point is the frequency of the wave. On the other hand, if we concentrate on the wavefront itself, and measure how far the wave progresses in one
second, we have the velocity of the wave. The third quantity, the wavelength,
is the distance between successive wavefronts. A little thought will convince
the reader that these three quantities are not independent; in fact, the velocity
is equal to the product of the other two.
For some types of waves, the velocity is different for different frequencies.
An example is ocean swell. These long wavelength waves are generated by
2 For example, in 1887 Heinrich Hertz discovered microwave electromagnetic radiation, as predicted
earlier by Maxwell.