Chapter 1
Nature of light
1.1Light – Wave or stream of particles? YES
1.1.1What is a wave?
More familiar forms of waves are sound, or waves on a surface of
water. In both cases, there is a perturbation with a periodic spatial
pattern which propagates, or travels in space. In the case of sound
waves in air for example, the perturbed quantity is the pressure,
which oscillates about the mean atmospheric pressure. In the case of
waves on a water surface, the perturbed quantity is simply the height
of the surface, which oscillates about its stationary level. illustration
1.1 shows an example of a wave, captured at a certain instant in time.
It is simpler to visualize a wave by drawing the “wave fronts”, which
are usually taken to be the crests of the wave. In the case of
illustration 1.1 the wave fronts are circular, as shown below the wave
plot.
1.1.2Evidence for wave properties of light
There are certain things that only waves can do, for example
interfere. Ripples in a pond caused by two pebbles dropped at the
same time exhibit this nicely: Where two crests overlap, the waves
reinforce each other, but where a crest and a trough coincide, the two
, waves actually cancel. This is illustrated in illustration 1.2. If light is a
wave, two sources emitting waves in a synchronized fashion 1should
produce a pattern of alternating bright and dark bands on a screen.
Thomas Young tried the experiment in the early 1800’s, and found the
expected pattern.
The wave model of light has one serious drawback, though: Unlike
other wave phenomena such as sound, or surface waves, it wasn’t
clear what the medium was that supported light waves. Giving it a
name – the “luminiferous aether” – didn’t help. James Clerk Maxwell’s
(1831 - 1879) theory of electromagnetism, however, showed that light
was a wave in combined electric and magnetic fields, which, being
force fields, didn’t need a material medium.
1.1.3Evidence for light as a stream of particles
One of the earliest proponents of the idea that light was a stream of
particles was Isaac Newton himself. Although Young’s findings and
others seemed to disprove that theory entirely, surprisingly other
experimental evidence appeared at the turn of the 20th. century
which could only be explained by the particle model of light! The
photoelectric effect, where light striking a metal dislodges electrons
from the metal atoms which can then flow as a current earned
Einstein the Nobel prize for his explanation in terms of photons.
We are forced to accept that both interpretations of the phenomenon
of light are true, although they appear to be contradictory. One
interpretation or the other will serve better in a particular context. For
our purposes, in understanding how optical instruments work, the
wave theory of light is entirely adequate.
1 When two sources of waves oscillate in step with each other, they
are said to be coherent. We will return to this when we study
interference phenomena in greater detail.
Nature of light
1.1Light – Wave or stream of particles? YES
1.1.1What is a wave?
More familiar forms of waves are sound, or waves on a surface of
water. In both cases, there is a perturbation with a periodic spatial
pattern which propagates, or travels in space. In the case of sound
waves in air for example, the perturbed quantity is the pressure,
which oscillates about the mean atmospheric pressure. In the case of
waves on a water surface, the perturbed quantity is simply the height
of the surface, which oscillates about its stationary level. illustration
1.1 shows an example of a wave, captured at a certain instant in time.
It is simpler to visualize a wave by drawing the “wave fronts”, which
are usually taken to be the crests of the wave. In the case of
illustration 1.1 the wave fronts are circular, as shown below the wave
plot.
1.1.2Evidence for wave properties of light
There are certain things that only waves can do, for example
interfere. Ripples in a pond caused by two pebbles dropped at the
same time exhibit this nicely: Where two crests overlap, the waves
reinforce each other, but where a crest and a trough coincide, the two
, waves actually cancel. This is illustrated in illustration 1.2. If light is a
wave, two sources emitting waves in a synchronized fashion 1should
produce a pattern of alternating bright and dark bands on a screen.
Thomas Young tried the experiment in the early 1800’s, and found the
expected pattern.
The wave model of light has one serious drawback, though: Unlike
other wave phenomena such as sound, or surface waves, it wasn’t
clear what the medium was that supported light waves. Giving it a
name – the “luminiferous aether” – didn’t help. James Clerk Maxwell’s
(1831 - 1879) theory of electromagnetism, however, showed that light
was a wave in combined electric and magnetic fields, which, being
force fields, didn’t need a material medium.
1.1.3Evidence for light as a stream of particles
One of the earliest proponents of the idea that light was a stream of
particles was Isaac Newton himself. Although Young’s findings and
others seemed to disprove that theory entirely, surprisingly other
experimental evidence appeared at the turn of the 20th. century
which could only be explained by the particle model of light! The
photoelectric effect, where light striking a metal dislodges electrons
from the metal atoms which can then flow as a current earned
Einstein the Nobel prize for his explanation in terms of photons.
We are forced to accept that both interpretations of the phenomenon
of light are true, although they appear to be contradictory. One
interpretation or the other will serve better in a particular context. For
our purposes, in understanding how optical instruments work, the
wave theory of light is entirely adequate.
1 When two sources of waves oscillate in step with each other, they
are said to be coherent. We will return to this when we study
interference phenomena in greater detail.
