Man's physical universe

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POLARIZED LIGHT 423

the crystal is doubly refracting. When light passes through an Iceland

spar crystal, it is split into two perpendicularly polarized beams. In

the case of tourmaline, one of these beams is absorbed completely

while the other one is only partially absorbed, so that the light obtained

is polarized. One can prove this by the following experiment: Place a

Polaroid disk (described below) over the calcite crystal and rotate

the disk.

One dot will disappear, and as the polarizer is further rotated,

this dot reappears and the other dot disappears. There is very little

absorption of light as it passes through the Iceland spar crystal.

When

light passes through a Nicol prism, only one of the beams of polarized

Fig. 187. The principle of the Polaroid screen. (Courtesy of The Polaroid

Corporation.)

light gets through.

Nicol prisms are used in polarimeters, which are

instruments in which the amount of the rotation of the plane of polarized

light as it passes through certain liquids or solutions is measured.

Polarimeters are used to measure the concentration of certain sugar

solutions inasmuch as the degree of rotation is proportional to the concentration

of the solution.

The new type of polarizing medium called

Polaroid, mentioned in

the introduction to this Section, has made possible widespread applications

of polarized light. A Polaroid screen consists of microscopic

quinine iodosulfate crystals closely packed in a transparent cellulose

plastic that is stretched before it solidifies, thus causing the crystals

to be oriented end to end and providing optical slots through which

light which is vibrating in only one plane may pass.

In 1941 Edwin Land produced a new polarizing medium, which uses

The new polarizing sheets

individual molecules rather than crystals.

are produced by stretching heated polyvinyl alcohol plastic, thus

aligning the plastic molecules, and then allowing the plastic to imbibe

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