Light is scattered when it meets a particle of similar size to its own wavelength. This can be seen in the scattering of sunlight by dust in the atmosphere. In 1847 Brucke showed that the scattering is proportional to the fourth power of the frequency of the light meaning that blue light is scattered more than red.
The scattering of sunlight as it passes
through the atmosphere accounts for the blue of the sky in daytime and the redness at sunset. As
sunlight travels through the atmosphere the light is scattered by the molecules of air and by the
tiny particles of dust and water vapour. Since the blue light is scattered more strongly then the red
the sky in daytime looks blue.
As the sun sets you are viewing the sunlight after it has
travelled a large distance through the atmosphere and so much of the blue light has been
scattered out leaving the characteristic red glow of a sunset. The photograph shows a sunset over
the Southern Ocean seen from Eastern Tasmania.
The effect is much more marked
after a volcanic eruption where large quantities of dust have been thrown up into the atmosphere.
There were superb sunsets after the eruptions of Mount St Helens and Mount Pinatubo in the
Philippines.
In a lunar eclipse the Earth's shadow covers the Moon but sunlight still
passes through the outer atmosphere of the Earth and falls on the Moon. The scattering of the
light in the Earth's atmosphere accounts for the redness of the Moon during the eclipse.
The effect of the scattering of sunlight can
also be seen in the Blue Mountains of New South Wales in Australia. There are many gum trees in
the area and the air is full of the vapour emitted by these trees. The small particles of gum oil in
the atmosphere scatter the light giving distant views of the mountains their distinctive blue
colour.
It also explains why most babies have blue or blue-grey eyes at birth. The
scattering is more marked from smaller particles such as the shorter molecules in the irises of
babies; in many children these join up as they grow and the colour of the iris alters from blue or
blue-grey to brown.
The beutiful turquoise colour if many of the lakes in the Canadian Rockies can be explained by the scattering of light from minute particles of rock held in suspension in the water.
Light is not only
scattered when it passes through a fluid but it is also polarized (see Figure 1). Two scattered
beams are shown at right angles to each other. The two beams are also plane polarized in
directions at right angles to each other.
The effects of the
scattering of light can be shown in the laboratory by passing a strong beam of light through a tank
of water to which a few drops of milk have been added, producing a suspension of tiny globules of
fat which scatter the light (Figure 2). Light scattered in this way is plane-polarised in two directions
at right angles to each other.
