Man's physical universe

xanabras

UNIT VI

SECTION 4

THE INVENTION OF POLARIZING-SCREENS PUT

POLARIZED LIGHT TO WORK

Introduction.

The first recorded discovery of polarized light was made by Bartholinus

of Denmark in 1670, who observed that an object viewed through

Iceland spar appeared double. Bartholinus reasoned that the crystal

separated the light into two beams. The later discovery of the physicist,

Nicol, that Iceland spar crystals could be cut at certain angles and

cemented together with Canada balsam to form prisms that would

eliminate one of the above two beams of light made possible the polarimeter

and polarizing microscope and thus harnessed polarized light

for laboratory use.

In 1852 W. D. Herapath discovered that quinine iodosulfate crystals

would polarize a beam of light, but he did not succeed in mounting

them in a satisfactory manner. It was the discovery of Edwin H.

Land, publicly announced in 1934, that these crystals could be suspended

in a plastic cellulose acetate film and aligned by stretching the

film, which made possible the polarization of light economically on a

large scale.

Polarized Light Is Light Vibrating in One Plane Only.

Light waves are transverse waves, that is, they vibrate at right angles

to the direction that the light is traveling, just as waves set up in a pool

of water by a stone thrown into it travel at right angles to the up-anddown

direction in which the water surface moves. Ordinary light is a

mixture of transverse vibrations in all possible directions. When light

is vibrating in one plane only, it is said to be plane polarized, or just

polarized.

Light May Be Polarized by Double Refraction in Crystals.

Light may be polarized by passing it through crystals of certain substances

such as Iceland spar or tourmaline. When a piece of Iceland

spar is placed over a dot, two dots are seen through the crystal because

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