# 3d Glasses/polarisation

1. Aug 4, 2010

### BkBkBk

I just got back from the cinema an hour ago and brought my 3d glasses home,i know these glasses work by only allowing light that oscillates in certain dirrections through the lenses,and each lense lies perpendicular to the other,allowing only one set of light through each lense,making you to see a composite image of two different images.

now what puzzled me was,when i look in the mirror with one i closed,the lense covering the open eye turns dark,so i can see everything else fine,but my eye and skin surrounding my eye disappears. in the mirror image.

what im wondering is,do mirrors polarise lightwaves,is that why i cant see my open eye but can my closed one?

could someone give me not only a qualatative explanation,but also inclusde some quantitative information so i get a feel for not only how the process works but how we describe it with mathematics?

2. Aug 4, 2010

### mgb_phys

No,
The point of the glasses is that they allow different polarizations through.
So the left one only lets through the image for the left eye and v.v.
If you look in a mirror with both eyes open then both eyes can see out through their own lens. But the left eye can only see back through the image of the left lens and the right eye through the right lens - so you can see both eyes.

But if you close your left eye, your right eye can see out through the right lens and can see back in through the right lens - but not back through the left one - so your left eye looks black.

3. Aug 4, 2010

### BkBkBk

but thats not what happens,when i close my left eye,i can still see it with the right eye,but the right eye cant see itself,the right lense is the one that goes black (the lense of the the open eye is the one that goes black)

4. Aug 4, 2010

### ACPower

Reflections do interesting things to polarization. In this case I guess the lenses do CW and CCW circular polarization, and the mirror reverses it.

5. Aug 4, 2010

### wildcatherder

That is correct. The more expensive, passive stereoscopic glasses use circular polarization. With simple, plane polarization, tilting one's head will immediately disrupt the stereo effect.

Circularly polarized light does reverse the sense of polarization upon reflection.

6. Aug 4, 2010

### BkBkBk

could this be analagous to overtaking a clock,ie watching a helix change dirrection?

7. Aug 4, 2010

### mgb_phys

ok - understand.
The RealD glasses used in cinema are circular polarizers.
They are made from a linear polarizer and then a quarter wave plate which rotates the linear polarized light 45deg and makes it circular.
When the light reflects it goes back through the quarter waveplate which rotates it another 45deg and then it hits the liner polarizer again at 90deg different to the original direction = blocked.