Why is the sky blue? Why are sunsets red? The answer involves Rayleigh scattering. When light strikes small particles, it bounces off in a different direction in a process called scattering. Rayleigh scattering is the scattering that occurs when the particles are smaller than the wavelength of the light. Blue light has a wavelength of about 400 nanometers, and red light has a wavelength of about 700 nanometers. Other colors of light are in between. A nanometer is a billionth of a meter. So, for Rayleigh scattering of visible light the particles must be smaller than 400-700 nanometers. Scattering can occur off larger particles, but it will follow a different scattering law.

The Rayleigh scattering law, derived by Lord Rayleigh in 1871, applies to particles smaller than the wavelength of the light being scattered. It states that the percentage of light that will be scattered is inversely proportional to...