Friday, April 23, 2010

Fun with polaroid shades

kw: observations, photographs

Many years ago I had a polarizing filter for the camera I had then, but I'd forgotten the interesting effects they have. I have also not worn any kind of sunglasses for at least thirty years, but recently got some at the insistence of my ophthalmologist. I saw a special on polarized clip-ons at the pharmacy and got a pair that fits my trifocals.

So today we were on a drive, and I noticed the stress patterns in the car windows. I just had to take a picture of them, so I held one lens of the shades in front of the camera lens and shot away. This first image is of the side window (my wife was driving).

My wife usually doesn't pay attention to such things, so I had to point it out to her, then she saw the patterns. We had a bit of a discussion of the way glass that has not been fully annealed has residual stresses in it that affect the polarization of light that passes through. Also that the light off the roadway was polarized because it was reflected. The combination produced this pattern when viewed through polarized sunglasses (which she was also wearing).

Then she mentioned that there was also a pattern on the back window. This image is a portion of that. Here the light from the sky is already polarized, as it is at most angles away from the Sun. The stress pattern in this window is quite different from the other.

She wanted to know why the windshield doesn't show any stress patterns. It is because there is no residual stress. The manufacturer takes great care to anneal windshield glass, which is one reason windshields cost so much.

Some other time I'll show her the colored effects you get from clear cellophane tape seen with polarized light (you have to put a polarizer on both sides of the tape, so it is easiest to stick it to a piece of glass first. Try overlapping pieces).

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