Scientists in the 19th century noted a phenomenon in greenhouses caused by the

opaqueness of glass to infrared radiation, which they called the greenhouse effect. The

glass in a greenhouse admits visible light from the Sun, which is then absorbed by the

soil and plants inside. They reradiate the solar energy as longer infrared waves, which

cannot pass back out through the glass and so help warm up the greenhouse. The

same phenomenon occurs on a vaster scale in the atmosphere as gases like methane

and carbon dioxide trap solar energy near the Earth’s surface.

In contrast to infrared radiation, higher frequency radiation such as visible light does not

resonate thermally with atoms or molecules, but may resonate with the electrons of the

atoms of a substance. In glass, visible light experiences much less reduction in the

amplitude of its waves than infrared radiation does, and most of its energy passes

through the glass quite easily. Atoms with resonant electrons that do absorb energy

from a light wave quickly pass on that energy by re-emitting it as radiation of the same

frequency to other atoms, which in turn pass it on to their neighbors.

This chain of absorptions and re-emissions, called forward scattering, follows a path

close to the light’s original direction of travel. A beam of light that strikes a pane of glass

will reach the “last atom” on the far side of the pane in an extremely short time. We see

the light after it emerges, and think of glass as transparent.

This process does slow the transmission of the wave, which is why light travels slower in glass than it does in air or a vacuum (a fact captured

numerically by the index of refraction of glass). For instance, light travels through a typical piece of optical glass at about 2/3 of its speed in a

vacuum. Of course, 2/3 of the speed of light in a vacuum is still a rather rapid pace....

If atoms of certain substances, such as cobalt, are added to glass, they may absorb certain frequencies of light without re-emitting them. Cobalt

glass has a deep blue-violet color, which indicates that all the lower visible frequencies (from red through green) are absorbed and cannot pass

through it. Substances which absorb all frequencies of visible light are called opaque.

Glass transparent to visible light

Waves absorbed by electrons of atoms

Re-emitted from neighbor to neighbor

Waves pass through but are slowed

30.8 - Polarization

Polarized wave: A

transverse wave that

oscillates in a single plane.

Polarized radiation: A form

ofradiation in which the

electric field of every wave

oscillates in the same plane.

Polaroid sunglasses like the ones shown above reduce the amount of light that passes

through them. How do they do it? They only let through light waves whose electric fields

oscillate in a certain plane. When a source such as the Sun emits light, waves emerge

whose electric fields vibrate in every plane parallel to the direction of propagation. This

is shown in Concept 1. The electric field of each individual wave does oscillate

consistently in its own plane, which is called the wave's plane of polarization, but the

radiation as a whole does not have this property.

The electric field of any electromagnetic wave is a vector quantity. A polarizing lens or

filter works by only letting through a certain component of the electric field of every light

wave that strikes it. An ideal polarizing filter can be visualized as a set of narrow parallel

slits whose direction is the filter’s transmission axis.

When a wave passes through the filter, the component of its electric field parallel to the

transmission axis is what passes through. The component perpendicular to the axis is

absorbed. As a result, waves that oscillate parallel to the slits pass through unhindered,

while waves that oscillate perpendicularly to them are completely absorbed.You may want to consider an analogy: a rope passing through a

gap in a picket fence. If you shake the rope vertically, the wave you create passes through unhindered. If you shake the rope horizontally, the

wave collides with the pickets, transfers energy to them, and does not pass through. In the case of a wave passing through at an oblique angle,

the component of its transverse displacement along the “picket axis” passes through while the fence absorbs the other component.

This basic model of how polarization works is shown with an ideal polarizing filter in Concept 2. In the diagram, the transmission axis happens

to be vertical. A “slit” allows waves oscillating in a vertical plane to pass through. For waves that oscillate in other planes, only the vertical

component of the electric field of the wave can pass through. Waves with a horizontal plane of polarization cannot get through the slit at all.

Ordinary light consists of waves whose electric fields are randomly oriented in all lateral directions. This is called unpolarized radiation. Two

common sources of unpolarized light are the Sun and incandescent light bulbs. If the radiation is created or filtered so that it has only waves

oscillating in a single plane, then it is linearly polarized. In the illustration in Concept 2, the polarizing filter is exposed to unpolarized radiation,

and it transmits linearly polarized radiation.

Polaroid sunglasses block some light waves.

Unpolarized light

Radiation can oscillate in many planes

(^564) Copyright 2000-2007 Kinetic Books Co. Chapter 30