W9_parallel_resonance.eps

382 Week 11: Light

white light

violet (largest n)

red (smallest n)

Figure 150: A prism causes violet light to be bent more than red light ateach interface, splitting
up the originally white incident light into a full spectrum.

In figure 150 the way a prism acts on an incident white beam of light is crudely repre-

sented. Red light, with the smallestn, is bent the least (at each of the two surfaces).

Violet light, with the largestn, is bent the most.

Similarly, water droplets or ice crystals that are all roughly the samesize can individ-

ually preferentially divert different colors of light into different angles, creating aring

spectrum around a white source seen through e.g. a falling rain. When the white source

is sunlight shining through raindrops in the early morning or late evening (so it can come

in underneath the raincloud cover) one sees only half of the ring, arainbow^106. When the

white source is sunlight shining through ice crystals in light clouds in theatmosphere,

one can get “sunbows”, or more rarely, “sun dogs” formed from refracting/reflecting off

of planar ice crystals.

11.4: Polarization

As we saw in the last chapter, the electric and magnetic field vectorscan point in two

independent directions perpendicular to the direction of propagation (the Poynting vec-

tor direction). We describe the behavior of the two components oftheelectricfield

component for a given fixed harmonic frequency as thepolarizationof the harmonic

wave. There are several ways to describe the polarization, and several physical processes

produce polaraized light.

Unpolarized Light

Unpolarized light is light for which the polarization vector is constantlyshifting its

direction around. For a few tens to thousands of wavelengths theelectric field vector

points in some direction. Then it suddenly shifts into a new direction, as its source gets

randomly interrupted. Unpolarized light is typically produced by “hot” or “random”

sources such as the Sun, a hot lightbulb filament, the gas in a fluorescent bulb, a candle

flame. On average, unpolarized light has its energy/intensity equallydistributed between

the two independent directions of polarization.

Linear Polarization

Linear polarization occurs whenever the electric field vector oscillates consistently in a

single vector direction in the plane perpendicular to propagation. The following are all

(^106) Or, more rarely, adouble rainbow! All the way across the sky!
I’ve never seen atriple rainbow, but they too are possible, and I’m guessing an easy way to go viral if you ever
capture one in a sappy video...