Suppose you have a long rope attached to two supports, and
you give one end of this rope a quick, up-and-down jerk. You would instantly
set up little waves in the rope which would travel from one end of the rope to
the other and back again.
Now it is obvious that if this rope passed through a narrow
slot as indicated. The waves would pass through the slot. But if the slot is
turned at right angles to the wave, it would immediately stop these waves as
soon as they hit the slot.
If you have millions
and millions of tiny ropes with little waves set up in them and each wave went
through a little slot, and all the waves were travelling in the same way, each
wave would pass through its respective slot.
If now all little
slots were instantly turned at right angles to their waves, the waves would all
stop as soon as they reached the slots. This is exactly what happens to light
waves when they pass through certain transparent substances.
The light waves going through a substance like Iceland
spar (calcite crystal) are immediately “slotted” and when they emerge they travel up and down or horizontally parallel with each other. The light
emerging from this Iceland spar is then said to be polarized.
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| Iceland spar (calcite crystal) |
A similar piece of Iceland spar placed with its axis at right
angles to the first piece of Iceland spar will have the same effect as the
slots have when you turn them at right angles to the rope wave, and a shutting
out of light will be the result.
The familiar substance known as Polaroid is the most obvious
use of this effect. It is an extremely ingenious process that coats cellophane
with tiny quinine crystals electrically distributed so that the molecules form
billions of up-and-down microscopic invisible lines and when light passes
through them it goes through the same change as when it passes through Iceland
spar. When another piece of Polaroid is placed with its axis at right angles to
the first piece the light is almost completely shut out, just as it was in the
case of Iceland spar.
You may recollect the “3-D” motion pictures which were quite
a fad a few years back, and the cardboard-framed spectacles you wore to see
another use of Polaroid. To see and object in the three
dimensions, we must see two images that differ slightly from the image as seen
through the right eye and the other image as seen through the left eye.
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| stereopticon |
The old stereopticon pictures of the 1890s illustrated this
principle. The picture taken “through the right eye” lens and the picture taken
“through the left eye” lens are slightly different and if they are thrown on
the screen together they will produce a smudge or blur because they do not
exactly coincide.
Now the trick is to get the right eye picture to go into the
right eye and not be seen by the left eye, and vice versa. This is done using polarized glasses.
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| stereopticon pictures |
In three-dimensional motion pictures, either in colour or
black and white, the pictures are taken through a double-lens camera and
projected through Polaroid lenses, the first lens being vertical and the second
horizontal. The two images are thrown on the screen and cause a blurred picture
because one image has been polarized horizontally.
To look at this without glasses would convey a blur, but as
soon as the glasses are worn, the right eyeglass which is Polaroid placed
vertically, permits the right eye picture to go only into the right eye and
shuts out the left eye picture because it is polarized at right angles to the
right lens.
The same thing is true for the left eye Polaroid on the
left eye glass is horizontal and permits the left eye picture to enter it, but
shuts out the right eye picture. In this way, we get the right eye image going
into the right eye only and the left eye image going into the left eye only,
with a consequent three-dimensional result.
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science