Sunlight, or visible light, is composed of the rainbow colors: red, orange, yellow, green, blue, and violet. Visible light is a part of the electromagnetic spectrum and carries energy in waves. Colors towards the red end of the spectrum have longer wavelengths and colors near the violet end of the spectrum have shorter wavelengths. Light waves can transfer energy when they interact with matter.
The molecules and other particles in Earth’s atmosphere scatter (re-emit) some wavelengths of light more easily than others. The shorter wavelengths, such as violet and blue are the most likely to be scattered. The sky is blue because our eyes are not very sensitive to violet light.
When the Sun is low in the sky, sunlight travels through a much greater thickness of atmosphere than it does when it is overhead. Over this greater distance, more wavelengths of light are scattered including longer wavelengths like green, yellow, and orange. Only the red light comes through to your eyes; so, the setting Sun often looks red.
In this activity, the suspended particles of milk scatter light like molecules and other particles in Earth’s atmosphere. Where the light has only traveled through the top layer of water, it appears light blue. Where it has traveled through most of the water, it appears yellow, orange, or red. If you add too much milk to the water, the glass will have a yellowish hue just like the atmosphere on a smoggy day.