How Do Polarizing Filters Work?

[darrenw91]

I've looked into polarizing filters a bit but I'm still not clear on what's actually happening and I was wondering if someone could help clear it up. Does a polarizing filter (such as on a camera or in sunglasses) block a specific mode of polarization and let all others through... or does it instead let only one mode pass and block all others? In other words, when light with a mix of all polarizations passes through a polarizer, what comes out the other side?

Thank you kindly!

 

[davenn]

hi Darren
welcome to the PF forums

polarising filters are normally designed to allow transmission of light polarised in one particular plane only
eg horizontal or vertical

some polarising filters for cameras have been designed to produce circular polarisation of the light

Dave

 

[A.T.]

Does a polarizing filter (such as on a camera or in sunglasses) block a specific mode of polarization and let all others through... or does it instead let only one mode pass and block all others?

It let's all components trough, which combined give you a polarized wave (e.g. in one plane):

https://www.youtube.com/watch?v=ycY2mUZHS84

 

[Andy Resnick]

I've looked into polarizing filters a bit but I'm still not clear on what's actually happening and I was wondering if someone could help clear it up. Does a polarizing filter (such as on a camera or in sunglasses) block a specific mode of polarization and let all others through... or does it instead let only one mode pass and block all others? In other words, when light with a mix of all polarizations passes through a polarizer, what comes out the other side?

Thank you kindly!

There are different kinds of polarizing filters. Crystal-based prism devices (for example, Nicol, Glan-Thompson, etc) use a combination of birefringence and total internal reflection to reject one polarization and pass the other. Thin-film polarizers (for example, Polaroid film used for camera filters, sunglasses, etc) use aligned long-chain polymers decorated with iodine to absorb one component and transmit the other, similar to wire-grid polarizers used in infrared/THz/microwave applications. Circular polarizers, used for autofocus cameras and 3D movies, add a quarter-wave retarder to either convert linear polarization to circular polarization (camera filters) or convert circular polarization to linear polarization (3D glasses).

 

[sophiecentaur]

I've looked into polarizing filters a bit but I'm still not clear on what's actually happening and I was wondering if someone could help clear it up. Does a polarizing filter (such as on a camera or in sunglasses) block a specific mode of polarization and let all others through... or does it instead let only one mode pass and block all others? In other words, when light with a mix of all polarizations passes through a polarizer, what comes out the other side?

Thank you kindly!

Perhaps you need some 'supplementary' information to help you with the basics. Some web pages miss out stuff that's necessary to grasp the first steps in this.
You need to understand what the polarisation of an EM wave actually represents. Ordinary light is not 'polarised' at all because it comes from many independent sources (say the hot surface of a glowing light bulb) A single EM wave, say from a familiar radio transmitting antenna will have its Electric field aligned with the 'rods'. If the rods are vertical, we say it is Vertically Polarised (VP) and the wave has one plane of polarisation all the time. (Likewise for HP)

A receiving antenna will pick up the maximum signal from a transmitting antenna if it is aligned with the field. As you tilt the receiving antenna it picks up less signal because it is only picking up the component of the transmitted signal that is parallel with it (zero when crossed). We're talking vectors here, like the triangles of forces and velocities. Vectors can always be 'resolved' into components.
A 'plane' polarising filter for as camera has an array of what are, effectively, minute antennae, which pick up one particular component of each of the many, randomly polarised waves that arrive and pass it on. So it does not just 'select' certain waves to let through. If it worked that way, it would only pass a tiny proportion of the incident energy; the rest would be stopped. In fact, it let's through half the total amount of energy through - most from some waves and a small amount from others - adding up to half. Another polariser, at right angles, would let the other half through. One linear filter, followed by a 'crossed' filter, will let nothing through.

That is the simplest (cheapest, these days) form of polariser, producing 'plane polarised' waves. It is possible to have 'Circular Polarisation' too. That is more complicated but it can be thought of a light in which the plane of polarisation rotates around the axis as the wave progresses. (The E vector follows a sort of corkscrew pattern) This type of polarisation is used in 3D Cinema, one of the lenses passes clockwise polarised light whilst the other passes anticlockwise polarised light. This means that, whatever angle you hold your head, you see the Left and Right pictures in the correct eye. If you used two plane polarisers then you would have to keep your head dead level or you would see both pictures with each eye.