Scattering of light and the colour of the sky
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.
Sunset over the Southern Ocean
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 Blue Mountains
polarised scattered light
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.
Figure 1
polarised scattered light
1
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.
Figure 2
Water with a few drops of milk added
2