Figure 1-4. Unpolarized and polarized light
When light vibrates in only one direction (fig 1-4b), it is said to be polarized.
b. Unpolarized light shining on a smooth surface is reflected as polarized light. One part of the
unpolarized light vibrating in a particular direction strikes the surface at an angle that causes the
reflection. The light reflected back from the surface vibrates in only one direction and is therefore
c. Polarized light is reflected from glass, water, highly polished surfaces, and even a clear blue
sky. Because the light is polarized it can be controlled by a device called a polarizing screen.
Polarizing screens filter polarized light.
a. Polarizing screens, or polarizing filters, are transparent to light polarized in one direction and
opaque when the direction of polarization is rotated 90 degrees. Thus, if you rotated the polarizing
screen to the proper position, you can either pass or filter out polarized light. Because normal light is
unpolarized, a polarizing screen will only reduce the amount of normal light.
b. Polarizing screens have a neutral density, that is, they have the same effect on all colors of
light. Blue light passes through just as well as red or any other color as long as the polarization is the
same. For this reason polarizing can be used with black and white or color film.
c. A polarizing screen is made of rodlike crystals lined up and embedded in plastic. The light
gets through only when the vibration (polarization) is in line with the crystals. Although the actual
physics are a little more complex than this, you can think of the light as a coin going into a narrow slot.
The coin fits through the slot only when the coin and the slot are in line. To get them in line you can
either rotate the coin or the slot.