Scattering of light by individual molecules, and by small droplets.
Scattering of light by small particles which scatter specific frequencies.
Our eyes give us most of the
information that we receive about the external world, and our
eyes depend on scattered light... the light from various sources
strikes the objects around us, and scatters back into the
environment, and some of that scattered light is intercepted and
absorbed by the backs of our eyes... the retina. Since the
middle of the 19th Century, physicists have been trying (with
great success) to understand the various processes by which
light is scattered. So let's discuss some of the most important
of these.
| Scattering of light by individual molecules, and by small droplets. |
Scattering of light by small particles which scatter specific frequencies. |
 |
 |
Many important molecular processes involve so-called “virtual states.” These are not eigenstates of the system, they are places where excitation energy can be very temporarily stored within the system, as allowed by the Uncertainty Relations. The virtual states are extremely short-lived, in accord with ΔEΔt ≃ ℏ, with Δt approaching zero.
☼ RAYLEIGH SCATTERING---Why the sky is blue! Gases are transparent, so why don't we see through the earth's atmosphere directly into the black of outer space? Because atoms or molecules of the gases of our atmosphere scatter incident photons in all directions, elastically. The probability of scattering goes like the photon kinetic energy to the fourth power, or the frequency of the light to the fourth power, so it is the highest frequency our eyes can easily see, namely blue, that dominates the scattering.
☼ MIE SCATTERING--- is elastic scattering of light from particles or droplets that have about the same size as a wavelength of light... the probability of scattering is independent of frequency, so white incident light is scattered as white light. The scattering is almost entirely in the forward direction, along the path of the incoming light. Mie scattering is what makes clouds white, and is also the reason that any powder ground fine enough appears white, no matter what color the bulk material preferentially reflects.
☼ RAMAN SCATTERING--- is technologically perhaps the most useful type of light scattering, because it offers a quick and reliable way to determine the molecular composition of an unknown sample of material. There are two types of Raman scattering, “Stokes” and “Anti-Stokes,” in which the scattered light has a lower or higher frequency than the incoming light. Raman scattering is weak, and has to be carefully distinguished from the overwhelmingly predominant Rayleigh scattering.
 |
 |
 |
Raman's interest in the scattering of light by matter was stimulated by his wondering why the ocean looks (usually) blue. His investigations of scattering led to his discovery of what we now call Raman scattering, but Raman scattering is not why the ocean looks blue. It looks blue because the water molecule has bands that absorb strongly in the infrared, red and yellow, but not blue. So the light scattered by water molecules is predominantly blue, that's the “natural” color of liquid water!
