There is probably no more widely misunderstood photographic accessory than the polarizing filter and there is often confusion about how linear polarizer film, what the difference is between them and when a circular polarizer is needed.
So first let’s look at polarization in general. You can think of light as traveling in waves. One way to visualize these waves is to imagine taking a length of rope and tying one end to a post. Moving the free end up and down sets up a “wave” along the rope which also moves up and down. If you think of the rope as representing a beam of light, the light would be “vertically polarized”. If the free end is moved from side to side, a wave that moves from side to side is set up. Again, if this was a light beam, you could call it “horizontally polarized”. Both of these are examples of plane or linear polarizing film.
You could also move the free end around in a circle and, if you got the timing just right, you could set up a wave that looks like a corkscrew. This is analogous to a light beam having circular polarization. You could move the free end of the rope around in either a clockwise or anticlockwise circle, corresponding to what are referred to as right handed or left handed circular polarization. If you mix some linear with some circular polarization you get what’s called elliptical polarization.
Of course this is a simplified explanation of polarization, but it’s not a bad analogy and should help you to visualize what’s going on. Technically, polarized light is light with its electric vector oriented in a direction which is predicable rather than random.
A linear polarizer is a device which selectively allows the passage of only certain orientations of plane polarized light. At one orientation it might allow the passage of only vertically polarized light, while if rotated by 90 degrees it would allow the passage of only horizontally polarized light. Half way in between, at say 45 degrees rotation, it would allow passage of only 45 degree plane polarized light.
You might ask what the use of such a device is? Well, most light sources (except for some lasers) put out randomly polarized light, i.e. an equal mixture of every kind of polarization. If objects reflected this light with no change, all a polarizer would do would be to act like a neutral density filter, no matter how it was oriented, but that’s not what happens.
Do you need a polarizer?
Actually, polarizers are one of the few universally useful filters. They’re effective for both film and digital and unlike warming and other color correcting filters, they really can’t be digitally simulated. They can increase color saturation, reduce reflections, and darken a blue sky. I’d say that if you own only one filter, it should probably be a polarizer. It’s the only filter I routinely carry as part of my general shooting kit.