The blue of a clear autumn sky (left) is the result of scattering by air molecules. The grey of fog (right) is the result of particle scattering by water droplets.

In this space shuttle photo the sun is below the horizon, but the atmosphere scatters light towards the camera. Above the atmosphere there is no scattering, so the sky is black.

What is scattering?

In a beam of direct sunlight all the photons travel in the same direction. Scattering occurs when photons interact with tiny particles in the air or water (or on the surface of objects) and then carry on in a new direction.

Unlike direct sunlight, or light reflected from a smooth surface such as a mirror, scattered light is diffuse - it comes from many different directions.

Light travelling through the atmosphere or through water is scattered in two different ways: by air or water molecules (molecular scattering) or by small particles suspended in the water (particle scattering).

Left: Clear seawater looks blue because the water scatters sunlight, and blue wavelengths are scattered more strongly. Without scattering the light would disappear into the deep, and the ocean would look black. Right: The 'white water' of a coccolithophore bloom is a result of scattering by tiny plants and chalk plates from the shells that surround them.

Molecular (Rayleigh) scattering

In molecular scattering the light interacts with air or water molecules*, which are tiny compared to the wavelength of the light. Molecular scattering has two characteristics:

1. Short wavelengths (violet and blue) are scattered much more than longer wavelengths (see the curve for air and water in figure 1).
2. The light is scattered more or less equally in all directions (see the curve for seawater in figure 2a, and the figure showing molecular scatter, figure 2b).

This explains the blue colour of the sky: the blue wavelengths are scattered away from the beam of sunlight, so it reaches you from all directions of the sky. It's the same with the sea: blue light is scattered back up towards the surface to a greater extent than other colours. (Water also absorbs red light more strongly than blue and green, but that is another story.)

* In reality only a little of seawater scattering is caused by the light interacting with single molecules. Instead random movement of the molecules (Brownian motion) gives rise to fluctuations in the density of the water: the number of molecules in a small volume of water is slightly different from the number in the neighbouring volume. These density differences occur on a scale that is larger than single molecules, but still very small compared to the wavelength of the light.

Particle (Mie) scattering

As the name implies, particle scattering is caused by the light interacting with particles that are suspended in the air or water. In the atmosphere this is usually water droplets in clouds and fog, or dust particles in haze. In the sea, or in rivers and lakes, the particles can be microscopic plants (phytoplankton) or sand and clay particles (sediment).

The particles are usually of a similar size to the wavelength of the light, or often much larger. Particle scattering has some characteristics that make it different from molecular scattering:

 

Molecular scattering
Particle scattering

Fig.2.  The volume scattering coefficient of different types of water (plotted on the left) describes the proportion of the light scattered at different scattering angles. 0° is the original direction, 90° is sideways, and 180° is light returning in the opposite direction to the original. For pure seawater the function is a flat line (equal scattering in all directions). Water with small particles scatter more light than seawater alone. The function gives higher scattering for all, angles, but the scattering is stronges in forward direction and decreases with scattering angle. Large particles scatter mainly in the forward direction. Here the 'small particles' are mineral grains less than 1 micron across. The 'large particles' are phytoplankton, over 1 micron across. Both are suspended in seawater at a concentration of 1 ppm

1. All wavelengths are scattered by roughly similar amounts; red light is scattered nearly as much as blue (fig.1).
2. Most of the light is scattered in a forward direction, much less is scattered backwards or sideways (fig.2).
3. The scattering is very much stronger than molecular scattering, and increases as the concentration of particles increases (fig.3).

This explains why clouds and fog appear white or grey (all colours are scattered). It also explains why everything appears blurred on a misty day (the light from anything you look at is scattered a little bit away from the original direction).

Finally it explains why you cannot see far in fog or in water with a lot of sediment: the light from anything distant is scattered away in all directions.

On misty days small water droplets in the air scatter light and make everything look blurred.

Last update: 28 November 2008

Contact: o4s@noc.soton.ac.uk