The Blueness of the Sky

Molecules of certain atmospheric gasses scatter some sunlight. The scattering process involves preferentially the short wavelengths of light, as are the molecules. Therefore, the light scattered by the atmosphere, the sky light, is blue. When the scattering particles are not small in comparison with the wavelengths, such as dust particles in the air, including condensation nuclei, the scattering involves more of the longer wavelengths. Because there is always some dust in the air, the sky light also contains longer wavelengths such as green and red, although the color mixture still looks blue. The more numerous and larger the dust particles are, the more of the other colors are mixed with the blue, and the paler the blue becomes. For this reason, the sky is rarely seen as deeply blue in industrial areas as in rural ones. Also in humid air masses, in which condensation nuclei take on water vapor from the air, the sky is paler blue than in dry air masses.

You can imitate the process that causes the blueness of the sky by letting extremely small particles scatter sunlight or light from a very bright source (projector), looking at the scattering particles from a direction perpendicular to the light beam. Minute particles are contained in certain suspensions, such as soap in water (if the water is not too hard), milk in water, a weak silver nitrate solution, or "hypo" (photographic fixer).

Fill a glass container, such as a small fish bowl or bottle with smooth sides and rectangular horizontal cross section, with water and place a sheet of black paper behind it. Let a strong light beam fall on the container from the right or the left side. Put a few drops of milk into the water with an eye dropper, stir, and observe. Add more milk, stir, and observe the changes that take place. When the mixture shows a distinctly bluish color, look through it directly at the light source.

Repeat the experiment using a soap solution, made from a few soap shavings dissolved in a little hot water. Compare the results with the previous ones.

If you can get some from a photographic store. use hypo and silver nitrate, which shows the effect particularly well. If no ready-make hypo is available, we make a strong solution of sodium-thiosulfate in water and add a little vinegar.

Repeat the experiment with a corn-starch solution. There should be a difference in clolor between the starch and the other materials as the particles are larger.

Material: Glass container, preferably of rectangular cross section with smooth sides

Eye dropper

Some milk, soap, silver nitrate, photographic fixing solution, corn starch

Water, black pepper