Why do polarising filters only work in one direction?

The digital camera sports automation that adjusts exposure and focus. These mechanisms are likely dependent on semi-silvered mirrors. These work like mirrored sunglasses; they pass some light and reflect the remainder. When you mount a polarizing filter, it can be an impairment diminishing the effectiveness of this wonderful automation. Because mounting a polarizing filter is often desirable, we need one that will not do mischief.

The polarizing filter we use is actually two filters sandwiched together. The front facing filter is an ordinary polarizing linear screen. This one does the job. It mitigates reflections, cuts haze, and increases saturation. Because it darkens blue sky, white clouds are caused to stand out in bold relief. So it’s the upfront filter that does the deed.

Behind the polarizing screen is mounted a second filter called a retarder. This filter effectively de-polarizes the light. This sandwich filter is now called a circular polarizer. That’s OK because the upfront polarizing screen’s actions are not impaired and neither is the camera’s automation. If you turn this “circular” polarizer around, you effectively mitigate its ability to polarize light.

By the way, early scientists working with materials that polarized light falsely assumed that light had a + and – component, something like a magnet has a North and South Pole. They assumed the filter somehow split light into positive and negative (polarized) light beams. This was proven false but the name polarization stuck.

Most would agree, the polarizing filter is a must have filter.

Polarisers are fun, and display quantum mechanics effects that can confuse people. Light, and all electromagnetic waves, can be considered to have two parts, with one that is a quarter wave behind the other. If the two parts have their electric fields aligned then we have a linear polarisation (and there are many directions of alignment). If the second part has turned to the side then the overall wave effect turns in a helix (spiral). The old filter selects just for the aligned waves, while the new filters select for the ones arranged as a helix (with a particular turn direction).

Many folk think that the linear aligned state is a 'quantum state' (i.e. countable) while in (quantum) reality it is only the two spiral states of left and right circular polarisation that are distinct countable states.

For a fun test, take three linear polarisers (e.g. polarised sunglasses at a store). The first set in a normal 'up' direction, the second set at right angles (across), and observe no light comes through. Now add the third at 45° at the back — still nothing, but now put that third polariser in the middle — Observe what happens, and wonder why! (Linear polarisation is not a quantum state..) See this MinutePhysics video for a demonstration.

The use of linear polarisers in quantum cryptography can fool people into thinking they have broken the laws of probability (see Bell's Theorem), when in fact it is just that the probability calculation are done on a circle not a square (those 45° linear polarisers are not a classic 50:50 selection!)

There are 3 kinds of polarising filters about:

• 1st Linea (for Manual focus film cameras).
• 2nd Circular (For manual focus film cameras), and
• 3rd Circular (for DSLRs and general autofocus cameras which contain both so as not to upset focussing and or exposure components).

Unfortunately some manufacturers don’t understand anything about them which is why your circular polarising was mounted in a rotating bezel when there is no point in rotating it.

You need to have a linear component in the circular polarising filter if you’re using a DSLR or auto focus camera.

So best to do the phone screen trick before you buy the filter. That way you can at least tell that it has a linear component even though it says CPL on the side.

DSLRs and digital cameras with "phase detect" autofocus have problems when fed with polarised light. So a "circular polariser" or CPL filter will, after polarisation, employ a "quarterwave plate" for turning linearly polarised light (after the polarisation proper) back into something that has polarisation in two directions. This effect is not independent of the light's wavelength, so it doesn't really create fully circular polarisation for all wavelengths, but somewhat elliptical polarisation. For the purposes of phase-based focusing and metering (including analog camera TTL flash metering), this is good enough, and the photographic process itself is not affected.

For a digital camera with only electronic viewfinder and merely contrast-based focusing, a CPL is not actually necessary and a linear polariser will be fine.

Now when you use the CPL in reversed orientation before a source of linearly polarised white light like a typical laptop screen, the non-perfect circular polarisation achieved after passing the quarterwave plane (which is only truly quarterwave at one particular wavelength of light) will mean that the orientation of the linear polariser behind it is not entirely unrelated to how the different wavelength portions of the white light manage to get through.

If you are using polarised sunglasses, you may get into trouble with the LCD screens on cameras or smartphones. When you are lucky, sunglass and screen producer have adopted polarisation conventions that differ by 45°, making the combination work reasonably either in landscape or portrait mode. Even better would be a quarterwave plate on the LCD or the polariser glasses. But I don't think that's customary.