Polarization – The interesting facts about polarization

There are only two kinds of wave motion. The first kind, longitudinal waves, has to do with sound and has already been discussed in the section dealing with sound. The other, having to do with light waves, is a transverse motion up and down or left and right wiggly motion. This is by far the most common wave motion and governs not only heat waves but radio waves, as well.

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.

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.

 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.

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.