30.9 - Scattering of light

Scattering: Absorption and

re-emission of light by

electrons, resulting in

dispersion and some

polarization.

The answer to a classic question í Why is the sky

blue? í rests in a phenomenon called scattering. In

this section, we give a classical (as opposed to

quantum mechanical) explanation of how scattering

occurs.

When light from the Sun strikes the electrons of various atoms in the Earth’s

atmosphere, the electrons can absorb the light’s energy, oscillating and increasing their

own energy. The electrons in turn re-emit this energy as light of the same wavelength.

In effect, the oscillating electrons act like tiny antennas, emitting electromagnetic

radiation in the frequency range of light.

An electron oscillates in a direction parallel to the electric field of the wave that

energizes it, as shown in Concept 1. The electron then emits light polarized in a plane

parallel to its vibration. We show a particular polarized wave that is re-emitted

downward toward the ground, since we are concerned with what an observer on the

surface of the Earth sees. Other light is scattered in other directions, including light

scattered upward and light scattered forward in its original direction of travel.

Scattering explains why we see the sky: Light passing through the atmosphere is

redirected due to scattering toward the surface of the Earth. In contrast, for an astronaut

observer in the vacuum of space, sunlight is not scattered at all so there is no sky glow:

Except for the stars, the sky appears black. To the astronaut, the disk of the Sun, a

combination of all colors, looks white. We illustrate this below:

The full spectrum combines to form white light.

The question remains, why is our sky blue rather than some other color? Light at the blue end of the visible spectrum, which has the shortest

wavelength, is 10 times more resonant with the electrons of atmospheric atoms than red light. This means blue light is scattered more than red,

so that more of it is redirected toward the ground.

Scattering also explains why we see the Sun as yellow rather than white. When you look up at the disk of the Sun from the Earth’s surface, the

bluest portion of its light has been scattered away to the sides. The remaining part of the Sun’s direct light appears somewhat yellowish.

Scattered sunlight gives the sky its blue glow.

Scattering

Light from Sun hits electron

· Electron oscillates

· Re-emits polarized wave to ground

· Shortest wavelengths scattered most

View from space

No scattering: sky is black

Sun appears white

Why the sky is blue (and the Sun

is yellow)

Shortest waves are scattered: sky is

blue

With blue scattered, Sun appears

yellowish white

(^566) Copyright 2000-2007 Kinetic Books Co. Chapter 30