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The answer to Q2 of the polarizing filters has been replaced by the FAQ on circular polarizers written by TOM DAVIS (davis@bedlam.asd.sgi.com)
DAVID JACOBSON (jacobson@cello.hpl.hp.com) suggested I should go for the wave theory on light instead of the particles. I tried. He also had some useful comments I have added to this FAQ, and he pointed out an error in A5 on the polarizers. I was wrong on the amount of light you loose, but it has been corrected now.
RICHARD KARASH (rkarash@world.std.com) also had some comments on the circular polarizer, but they were already in the circular polarizer FAQ by Tom Davis.
Via news JEFF SPIRER (jffspirer@aol.com) and Dr. GROVER LARKINS (larkinsg@solix. fiu.edu) also commented on the polarizers, mostly about the circular vs. linear question, and dr. Larkins commented on my advice of overexposing film when in doubt of the light. This is corrected. The other comments on the circular polarizer were already corrected in the circular polarizer faq by Tom Davis.
update september 29, 1994
LUCA DE ALFARO (luca@cs.stanford.edu) sent in a test about determining if you really need a circular polarizer. Jeff Spirer sent a comment on this, so I decided to unite both of them in Q6 and A6 on polarizers.
update october 3, 1994
JOE CALI (Joe.Cali@anu.edu.au) sent in an explanation of the so called Brewsters Angle. This is added to the FAQ in Q7 and A7.
update october 4, 1994
Rev. Dr. PHIL HERRING (revdoc@wumpus.uow.edu.au) pointed out that even if you use a manual camera, a linear polarizer might disturb your light meter. This is added to A2 on polarizers.
update november 28, 1994
A.T. Young (aty@mintaka.sdsu.edu) suggested that instead of shooting a roll of film just to see the effects of polarizers, we should take a look at the book "Polarized Light in Nature", by G.P. Können, which gives you a lot of information and shows you a lot without having to shoot even one picture. Of course, for those who cannot find the book (me, for example), keep trying yourself, or try to find another book on this subject.
Also, he gave the comment that at a refractive index of 1 there's no reflection, so 'air' should be removed from the list in Q7/A7. I did, but keep in mind all the refractive indices are relative to air (n=1).
Last, he pointed out that the sky is not reflected light but scattered light. This point is corrected in Q3/A3 on polarizers.
update april 19, 1995
I saw a thread about the advantages/disadvantages of using multiple filters at the same time, so I added Q6/A6 to the UV filters part. I tried to use all the comments I saw in the newsgroups, leaving the ones that were really wrong. You can see for yourself how well I managed doing this. Because the answer is a compilation of comments from different people, I can't give credits for it to one person.
update july 14, 1995
I noticed a question about the use of polarizers and non-uniformly coloured skies when a polarizer was used. I also read some comments on this that it might have been caused by vignetting, when you are using a wide-angle lens. I combined this question and comments in Q8/A8 on polarizers. Again, because the answers came from a number of people, no single person can get the credit for it.