< When wearing polarized sunglasses, what are the checkered patterns that
seem to show up on automotive tinted windows? Why can you only see them with
polarized glasses on, and only at certain angles in relation to the window?
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OK, someone infinitely better equipped to answer this question will probably be along shortly, but here’s what I can tell you:
Polarizing light filters act like a fence. Tall skinny things can get through, and short wide things can’t. So your polarized sunglasses might only let light oriented at 0 degrees or 180 degrees through; and they filter out light that is oriented at 50 degrees, or 90 degrees, and so on. If you angle your head at 30 degrees, the sunglasses will let light come through that is oriented at 30 degrees or 210 degrees, filtering out all 0 degree light.
Then the tint on your windshield is like another fence. When you view light through both sunglasses and tint, less light will reach your eyes. If you tilt your head back and forth, the amount of light let through will change as you tilt your head, so you see patterns.
I know that’s incomplete, but I hope it helps.
The way a polarizing filter works is by blocking light waves except for those that happen to be in a given plane. For polarizing sunglasses, this is generally the vertical plane, as glare tends to be mostly in the horizontal plane. When the planes are perpendicular to each other, the light is most effectively blocked.
I believe that plastics under stress tend to act kind of like polarizing filters, probably from the polymer chains lining up next to each other. Usually they only filter a certain color at a certain location. Looking at clear plastics under stress through polarizing filter is often used to see where the plastic is stressed.
For further fun, try using a digital watch with the sunglasses on. Play around with the orientation of the watch.
Tint is not a factor. Waterj2 is right about the stress. The stress can also be present in glass. When glass breaks, it tends to break where the stress is greatest. Safety glass used in cars and elsewhere is intentionally given this stress pattern so that it will shatter into small pieces when broken instead of long shards that could slice and stab.
I don’t have an explanation as to how stress causes this polarization on the molecular level.
I can say that to see the pattern, you must have polarized light going through the glass and then through a polarizing filter. So you have the polarizing filter, but why is polarized light going through the glass? There’s no polarizing filter on the other side.
Light can be polarized by reflecting at the correct angle from a shiny surface (not bare metal). This could be a source of the polarized light.
A second possibility involves the fact that when light enters the glass at an angle, part is reflected off the window and the rest is refracted through the window. It turns out that at the correct angle, both the reflected and refracted light are polarized. The refracted light could be the source of the polarized light that passes through the glass.
liquid crystal displays also work by polarization. if you turn your head while wearing those sunglasses, the display disappears.
Generally, polymers are made from long stringy molecules. I would guess that in areas of high stress, these line up parallel to each other. These strings probably interfere with certain wavelengths in certain planes, which then become noticeable when you view it through a polarizing filter.
MagicalSilverKey asked this exact same question two months ago, and got some good answers.
automobile windshields: a two part question
Check it out, might be helpful.