Light Scattering

When a few drops of milk are stirred into water, light is scattered and polarized.

Discussion

Here a beam of light passes upward through the center of a container of water. The light beam is scarcely visible from the side as it passes through water. When a few drops of skim milk are stirred into the water, the beam becomes visible from the side because of light scattered from the beam in that direction by casein (milk protein) micelles. The scattered light is somewhat blue in color, which means that blue light is more efficiently scattered than is light of other colors.

We have seen from our ripple tank experiments that objects scatter waves most efficiently when the dimensions of the object are comparable to the wavelength, and that there is little scattering when the dimensions of the object are much smaller than the wavelength. These results with milk suggest that the dimensions of the casein micelles are close to the wavelength of blue light, but smaller than the wavelengths of other colors of light.

What we see when a polarizing sheet is placed in front of the container and rotated shows that scattered light is polarized. This is a result of the mechanism by which light is scattered by the casein micelles. Light scattered from a beam by casein micelles suspended in water or by density fluctuations or particulate matter in air is polarized with its axis of polarization perpendicular to the direction of the light beam. On a day when the sky is not overcast, if you have a pair of polarizing sunglasses, it is easy to verify the direction of polarization of scattered blue light. When you wear polarizing sunglasses, the polarization axis of the lenses is vertical, so polarized light with its axis of polarization parallel to the polarization axis of the lens will be transmitted, but polarized light with its axis of polarization perpendicular to that of the lens will be absorbed. With your back to the sun, hold the sunglasses in front of your face in their normal orientation. Now close one eye and with the other look through one lens of the sunglasses at a patch of blue sky overhead. When you rotate the sunglasses 90° from their normal orientation you will see that the light intensity increases. This means that blue light scattered by the atmosphere is polarized at 90° to the direction of light traveling from the sun, as we have suggested. (This experiment can also be done with a sheet of polarizing material, if polarizing sunglasses are not available.)