# B How to find the orientation of polarization of a polarizer

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1. Nov 20, 2017

### boxfullofvacuumtubes

If you have a new linear polarizer, how do you find its orientation of polarization? In other words, how do you orient it so it would, let say, polarize light vertically if there is no marking for the 0 degrees angle?

2. Nov 21, 2017

### Orodruin

Staff Emeritus
The easiest way would be using a known linear polariser and place them in series or use a known polarised source of light such as the sky or an angled reflection.

Edit: Or the light from an LCD screen. A few years back I noted in delight that all LCDs in the bus stations where I lived had the correct polarisation orientation to be seen with polarised sunglasses (which are always oriented the same way to reduce reflections on horizontal surfaces) assuming you keep your head vertical. Whether by happy coincidence or intelligent design I cannot say.

Last edited: Nov 21, 2017
3. Nov 21, 2017

### boxfullofvacuumtubes

If I don't have another known linear polarizer to check the orientation against, is there any way to use a quarter wave plate with a known fast axis for this purpose? I just can't think of any, but smart people around here might.

Orodruin, I like your idea to use an LCD screen as a source of light with known polarization.

4. Nov 21, 2017

### nasu

The polarization of one of the LCD displays I have here is oriented at 45 degrees relative to the edges.
But you can always look at the light reflected by a shiny floor, at some angle. The reflected light is partially polarized parallel to the floor.
When the floor looks the darkest through the polarizer the direction of the polarizer is vertical (perpendicular to the floor).