# I'm seeing things ... **URL:** \ **Category:** Factual Questions\ **Created:** [June 1, 2003, 5:09pm UTC](https://boards.straightdope.com/t/im-seeing-things/178959 "2003-06-01T17:09:12Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![peritrochoid](https://avatars.discourse-cdn.com/v4/letter/p/c68b51/32.png) [@peritrochoid](https://boards.straightdope.com/u/peritrochoid)\ **Post date:** [June 1, 2003, 5:09pm UTC](https://boards.straightdope.com/t/im-seeing-things/178959/1 "2003-06-01T17:09:12Z")
On the recommendation of some friends and family, I recently bought a pair of polarized sunglasses for driving. Being an amateur photographer, I have used polarizing filters, so I understand the principle, but I have noticed one thing in particular that has me baffled. The windshield of certain minivans (almost exclusively in the General Motors family) appears reflective purple. I have even seen a Mercedes or two with similar reflective blue. I don’t see this when I remove my glasses. These sunglasses sharply reduce reflections and glare from all other vehicles, so why are these appearing to be reflective, and colored to boot? Is there a certain way the glass is manufactured that is causing this? Could these be polarized windshields, causing all kinds of havoc when viewing them through polarized sunglasses? Is this, perhaps, an “easter egg” feature the manufacturers include as a special treat for wearers of polarized sunglasses? This keeps me up nights … please help enlighten me so I may sleep. 😉 ---
**Author:** ![jjimm](https://avatars.discourse-cdn.com/v4/letter/j/ba8739/32.png) [@jjimm](https://boards.straightdope.com/u/jjimm)\ **Post date:** [June 1, 2003, 5:21pm UTC](https://boards.straightdope.com/t/im-seeing-things/178959/2 "2003-06-01T17:21:34Z")
Yes, they do polarize windshields, and that is what you’re seeing. You will now also see stress patterns in other clear materials. As to why, I think it’s to reduce glare reflected off slick roads, etc. I may be wrong. Welcome to the boards, btw. ---
**Author:** ![peritrochoid](https://avatars.discourse-cdn.com/v4/letter/p/c68b51/32.png) [@peritrochoid](https://boards.straightdope.com/u/peritrochoid)\ **Post date:** [June 1, 2003, 6:25pm UTC](https://boards.straightdope.com/t/im-seeing-things/178959/3 "2003-06-01T18:25:08Z")
Thanks for the info and thanks for the welcome, jjimm. I have noticed the stress patterns in certain materials like the current crop of plastic headlamp covers and aircraft windows. I’ve always been a very curious person, so by revealing this whole new world these glasses are worth every penny in entertainment value. 😉 To carry the question further, any idea why the polarized windshields are almost universally purple? ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [June 1, 2003, 7:56pm UTC](https://boards.straightdope.com/t/im-seeing-things/178959/4 "2003-06-01T19:56:55Z")
Ordinary car windows are not _built_ to be polarized. The layering of the safety glass results in the light bouncing inside the plastic layer to pick up some polarization. Polarized sunglasses allow you to detect this. Angles play a key role in this. Depending on the relative angles of the sun, the window, etc., affects the plane of polarization. (There’s a great story about this effect in Feynman’s “Surely you’re joking…” book.) The thickness of the thin layer of plastic sandwiched between the glass affects the colors and pattern you see. (Interference effects a la an oil slick.) Stress patterns also play a role. Google on “Brewster’s angle”. ---
**Author:** ![jjimm](https://avatars.discourse-cdn.com/v4/letter/j/ba8739/32.png) [@jjimm](https://boards.straightdope.com/u/jjimm)\ **Post date:** [June 1, 2003, 8:00pm UTC](https://boards.straightdope.com/t/im-seeing-things/178959/5 "2003-06-01T20:00:04Z")
Another weird effect of them is that, if you look at a clear sky through only one lens, then turn the glasses by 90°, the sky goes much darker. ---
**Author:** ![Shalmanese](https://avatars.discourse-cdn.com/v4/letter/s/45deac/32.png) [@Shalmanese](https://boards.straightdope.com/u/Shalmanese)\ **Post date:** [June 1, 2003, 8:24pm UTC](https://boards.straightdope.com/t/im-seeing-things/178959/6 "2003-06-01T20:24:25Z")
Sunlight which bounces of roads and flat land tends to be more horizontally polarised than vertically. The idea behind polarised glasses is that it reduces the amount of “glare” light while still keeping as much normal light as possible. If you wear them at 90 degrees, you should find that they will make the road look ver bright. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [June 1, 2003, 9:15pm UTC](https://boards.straightdope.com/t/im-seeing-things/178959/7 "2003-06-01T21:15:36Z")
> [@](#): > > \*Originally posted by jjimm \* > \*\*Another weird effect of them is that, if you look at a clear sky through only one lens, then turn the glasses by 90°, the sky goes much darker. \*\* This effect is often used by photographers to deepen the saturation of blue skies. ---
**Author:** ![rowrrbazzle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rowrrbazzle/32/414_2.png) [@rowrrbazzle](https://boards.straightdope.com/u/rowrrbazzle)\ **Post date:** [June 1, 2003, 10:34pm UTC](https://boards.straightdope.com/t/im-seeing-things/178959/8 "2003-06-01T22:34:01Z")
Actually, the sky’s maximum polarization is 90° from the sun, obviously decreasing away from that. BTW, Iceland spar (calcite crystal) can be used to detect polarization, and it’s conjectured that Vikings used it in navigation to determine the position of the sun. [http://www.polarization.com/viking/viking.html](http://www.polarization.com/viking/viking.html)