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. 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.

The wonderful emerald green of the rivers and the blue of the lakes in the Canadian rockies is due to the scattering of light from the particles of rock suspended in the water. The colour tends to be green rather than blue because the rock particles are larger than the molecules of the air and so scatter the longer wavelengths more strongly.

Scattering 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.

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.