Color I

Rayleigh Scattering

Rayleigh Scattering occurs if light encounters objects which are much smaller than the wavelength of this incident light. In addition, these objects must have their important resonances at frequencies greater than those of light.

Rule for Rayleigh Scattering

The shorter the wavelength of the incident light, the stronger the light is scattered.

For incident white light, the amount of scattered light with a wavelength of 400 nm is almost 10 times larger than the amount of scattered light with a wavelength of 700 nm.

The consequence of this scattering process is

BLUE SKY

Consider this sketch of sun and earth:

figure
  • A looks towards the left into the sky:
    Only scattering light reaches her eyes. Since short-wavelength light is scattered most,
    The sky looks blue.
  • B sees the sun setting: She sees direct rays from the sun. Scattering removed the blue end of the spectrum (this depends on how much atmosphere is traversed and how much dust is in the air).
    The sun appears red.

After volcano eruptions sunrises and sunsets can be very colorful. The reason is that there is a lot of dust in the air.

Why are clouds white? Water droplets are much larger ( about 50 times) than the wavelength of light.

When there are many surfaces to present to reflect light, almost all light is scattered. ==>

white appearance of the clouds

Of course: very dense clouds DO NOT transmit light.
They either absorb the light or reflect it upward.

black clouds

Experiment

Light scattering from milk

a little milk - "blue" sky effect
more milk - "white cloud" effect

Try it! Blue skies.

Experiment from the St. Louis Science Center

Ch. Elster
Aug 26 14:27:03 EDT 2019