Electromagnetic Radiation and Wave Behaviour 111
distance discussed earlier in connection with electric and magnetic
forces. The solution to these two related mysteries, provided by Faraday
and Maxwell, is the concept of a field. Later in this book, after we have
studied more about elementary particles and their basic interactions, we
shall return to this mysterious question and consider another possible
solution.
Although it is more difficult to conceive the wave nature of light than
that of the ocean because of the absence of a concrete medium, light,
nevertheless, displays exactly the same wave behaviour characteristic of
waves on the water. Let us consider two phenomena characteristic of
waves, namely, linear super-position and interference. If I drop a stone
into a quiet pond of water, a wave front with a circular shape will
propagate from the point where the stone enters the water. If two stones
are dropped into the water a short distance apart, two circular waveforms
will propagate and interfere with each other.
The two waves will flow through each other without affecting each
other, that is, after passing each other the two waves are exactly the same
as they were before, i.e., they retain their circular form. In the region
where they meet, however, they interfere with each other. The motion of
the water up and down due to the two waves will either add together or
subtract depending on whether or not two crests arrived at the same point
or a crest and a trough arrived at the same point. If two crests arrive at
the same place, then, the waves add such that a crest is created higher
than that of a single wave is created. If, on the other hand, the crest from
one wave arrives at the trough of another, then, the two waves can
momentarily cancel so that it appears there is no disturbance of the water
at all at this point. However, an instant later, as the two waves propagate
past each other, one observes the two waves again.
Light also can interfere constructively or destructively with itself just
like water waves. Let us consider light from the same source shining
through two slits of some opaque material. This situation is analogous to
the dropping of two stones since spherical light waves will emanate from
each of the two slits. If we now observe the light from these two slits
projected on a screen, we will observe a pattern of alternating illuminated
and dark patches. Those positions, which are illuminated, are the places
where the light from the two sources arrived in phase and the dark points
where the two beams of light arrived out of phase. It was the two-slit
interference experiment first performed in 1789 by Thomas Young,
which finally settled the controversy between Newton and Huygens
concerning the wave or particle nature of light.