Why the Sky is Blue and Sunsets are Red

Adding milk increases the amount of light scattered.

Discussion

Light passes upward through the suspension in the dish and is then reflected by the screen above. The color of the beam of light seen from the side of the container is the color of scattered light; the color of light reflected by the screen is that of the transmitted light that has passed through the suspension.

As more milk is added, the extent of scattering increases. This leads to a flaring out of the cylinder of scattered light and to a reddening of the light that is passes through the suspension of milk in water.

This selective scattering of blue light and transmission of red light explains why the sky overhead is blue and sunsets red. During the day, when we look skyward, the light we see is scattered blue light; if there were no light scattering, the sky would be black. In the evening, light from the sun must travel a greater distance through the atmosphere, so scattering by smoke and other particulate matter in the atmosphere removes short wavelength colors, leaving longer wavelength red light. This scattered light is polarized in the same way as light passing through a milk suspension in water is polarized.