Figure 16.20 shows a wave on a rope in several different situations.
In (a) the rope is vibrating only up and down-it is vibrating
in the vertical plane. We say the wave on this rope is polarized
in the vertical plane or that it is vertically polarized. In (b)
the rope is polarized in the horizontal plane. In (c) the wave
on the rope is not plane polarized. It may be the kind of wave
you create if you move the end of the rope in a circle-or, better
still, move the end of the rope transversely but at various angles
or directions. A transverse disturbance moves down the rope but
it is not plane polarized. In (d) a barrier with a vertical slit
is placed around the rope-somewhat like a picket fence. As the
rope moves back and forth the barrier absorbs or cancels the movement.
But when the rope moves up and down the barrier allows it to pass
freely. Beyond the barrier the wave on the rope is polarized vertically
as in (a). In (e) a barrier with a horizontal slit is placed around
the vibrating rope as was done in (d). This time as the rope moves
up and down the barrier absorbs or cancels the movement. But when
the rope moves horizontally the barrier allows it to pass freely.
Beyond the barrier the wave on the rope is polarized horizontally
as in (b). These barriers act as polarizers and allow us to selectively
create polarized waves from unpolarized waves.
Figure 16.21 shows a vertically polarized wave on a rope and a
vertical polarizer. The wave is allowed to pass through freely.
But a vertically polarized wave is stopped or absorbed by the
horizontal polarizer. Of course, a horizontally polarized wave
would be passed freely by a horizontal polarizer and blocked by
a vertical polarizer. There is nothing similar to this for longitudinal
Light is a transverse wave of electric and magnetic fields. Since
light is a transverse wave it may be polarized. By the nineteenth
century-but before the electromagnetic nature of light was known-it
had been found that light could be polarized. But light is not
always polarized. Light from a light bulb-or a candle flame or
the Sun or most ordinary sources-is a flood of tiny bundles of
light, called photons, that are emitted independently by thousands
of thousands of individual atoms. Each atom emits its light with
some polarization state. But the next atom is entirely independent
or random. We say that such light is unpolarized-just as the wave
on a rope in Figure 16.20(c). We can use a diagram like Figure
16.22 to indicate unpolarized light by indicating that vibrations
in all different directions are present. Just as with the wave
on a rope, unpolarized light can be passed through a polarizer
and the light that passes through will be polarized.
The most common way of obtaining polarized light today is by passing ordinary, unpolarized light through a sheet of Polaroid, or a polarizing filter. Polaroid, although quite common today, was invented in 1928 by Edwin H Land while still a freshman Physics major at Harvard (You may know him more for the Polaroid Land cameras that produce instant pictures). Polaroid is made of long molecules oriented parallel to each other. The electric field parallel to these molecules is absorbed but the electric field perpendicular to them is transmitted. Thus, Polaroid acts as a series of fine, parallel slits somewhat like a microscopic picket fence.
Figure 16.23 illustrates unpolarized light becoming polarized
as it passes through such a polarizer. Experimentally we can then
check to see if this light is, indeed, polarized, by passing it
through a second polarizer, now called an analyzer, as shown in
Figure 16.24. Rotating the analyzer will cause the intensity of
the polarized light to change from a maximum to zero. If light
is completely unpolarized, then rotation of a Polaroid analyzer
will have no effect on the intensity of the light (although it
will change the plane of polarization). If light is only partially
polarized, then rotation of a Polaroid analyzer will change the
intensity of the light but the minimum intensity will not be zero.
Polarized light can also be produced by reflection of initially unpolarized light from a non-metallic surface. The reflected beam will be at least partially polarized in the plane of the reflecting surface. When the angle between the reflected and refracted rays is 90 the reflected ray is completely polarized. This was found experimentally in 1812 by David Brewster, a Scottish Physicist.
This polarization of reflected light is very useful. Most reflections
of sunlight occur from nearly horizontal surfaces-a lake's surface,
the hood of a car, a road's surface. That means the glare of reflected
light will usually be partially polarized horizontally. If you
wear Polaroid sunglasses with the transmission axes of the lenses
vertical, much of the glare from reflected light will be eliminated.
That is why Polaroid sunglasses are effective. Figure 16.25 shows
a sketch of such a reflection. This polarization of reflected
light is useful to photographers who can use a polarizing filter
to control reflections from windows or the surface of water.
Skylight is polarized; you can easily demonstrate this by rotating
Polaroid sunglasses while looking at the sky and seeing the brightness
of the sky increase and decrease. This is most noticeable if you
look at an area of the sky that is 90 from the Sun. This can be
understood by looking at Figure 16.26. There sunlight is shown
striking an air molecule. The sunlight causes the molecule to
vibrate back and forth and reradiate another light wave. The reradiated
wave will be very strong in a direction perpendicular to the vibrations
and weak to non-existent along the direction of the vibration.
The reradiated light that is seen in the direction shown in the
figure will be polarized as shown. Skylight is actually the result
of many such scatterings so the polarization is not complete.
Why is the sky blue? The scattering from air molecules of sunlight
into skylight described in the previous paragraph is more efficient
for shorter wavelength light because it is a resonance phenomenon.
That means that it is the blue and violet light that is scattered
many times so that we see this light coming from all directions.
The longer wavelength light-the red and orange-continues on with
far less scattering. At sunrise and sunset, light from the Sun
travels through much more of our atmosphere so the light from
the Sun is a deep orange or red. This is illustrated in Figure
16.27. Indeed, Physics is everywhere!
Q: In general, would you expect the sky to be darker or lighter from a very high mountain?
A: On the Moon, with no atmosphere to scatter sunlight at all, the sky is always black. Atop a very high mountain (or from a very high-flying airplane), with less atmosphere to scatter sunlight, the sky will be darker-still blue, but a darker blue. Moisture and dust particles in the air greatly affect the way light is scattered. But, generally speaking, the sky is a darker blue from very high mountains.