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I have 3D glasses made up of plastic.

First case: When I hold front side of the 3D glasses (by this I mean that side on which light falls) in front of a computer screen, light coming from screen is not blocked, at any angle of rotation. But I see somewhat (slightly) bluish screen when 3D glasses are held horizontally; and yellowish (slightly) screen when glasses are held vertically.

Second case: Now I flip the side of the glasses i.e. "back" side of glasses is in front of screen and "front" side is closer to eyes. In this second case, when glasses are held horizontally, the light is blocked. When glasses are held vertically, light is not blocked; I can see the screen. For intermediate angle, intensity of light varies from minimum to maximum.

Questions:

  1. Why I get slightly colored screens in the first case and why light is not blocked ?

  2. Why light is blocked in the second case ?

I know (after reading somewhere) that glasses consists of a pair of circular polarizer and linear polarizer. Circular polarizer in my first case will be near to computer screen and linear polarizer will be near to eyes; and in second case vice versa.

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    $\begingroup$ You probably need to mention which 3D glasses in this case, since there are many types... I assumed polarized 3D system? $\endgroup$
    – Andrew T.
    Commented May 7, 2015 at 9:28
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    $\begingroup$ Perhaps you don't want me to refer you to this wikipedia article (why?), but it's really well explained there: en.wikipedia.org/wiki/… $\endgroup$
    – John Dvorak
    Commented May 7, 2015 at 9:44
  • $\begingroup$ @AndrewT. Yes. polarized 3D system. $\endgroup$
    – atom
    Commented May 7, 2015 at 10:05
  • $\begingroup$ @JanDvorak: I had read that article before I posted this question. I did not find answers to my 3 queries above, in that article. $\endgroup$
    – atom
    Commented May 7, 2015 at 10:07
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    $\begingroup$ Rather than stating "don't send me to Wiki" it would be better to say "I have read x, and it leaves me wondering about Y". It "shows research effort" which is one of the criteria for getting upvotes (and often, better effort in answers) $\endgroup$
    – Floris
    Commented May 7, 2015 at 11:57

1 Answer 1

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The two lenses in modern 3D glasses are designed to select the two circular polarizations. The left lens only transmits left-circularly polarized light and the right lens only transmits right-circularly polarized light (or vice versa).

The problem is that there is no material which acts as a circular polarization filter on its own. The way in which they are made is to stack a quarter waveplate and a linear polarizer together as shown in the diagram below. The quarter waveplate converts the circularly polarized light to linear polarized light, and then the polarizer either blocks or transmits it depending upon the original handedness of the circularly polarized light.

Schematic of a circular polarizer

A fundamental fact about the way LCD displays operate is that the light which you see is linearly polarized. So, what happens when you let the linear polarized light from your LCD screen transmit through in the usual direction is; the quarter waveplate converts the linear polarized light to circular and the polarizer then transmits half of the light, no matter what the initial polarization. This is why you see only slight variations when you rotate the orientation of the glasses with respect to the screen. The slight bluish/yellowish variation that you see is due to chromatic dispersion in the quarter waveplate. I.E. it does not perfectly convert linearly polarized light to circular for all wavelengths.

In the second case, where you flip the glasses, the linear polarizer is now in the front, and the glasses act just like a standard linear polarizer in front of a polarized light source. In one orientation they will block the linear polarized light from the LCD screen, and in the orthogonal orientation they will transmit all of it. The quarter waveplate, which is now after the polarizer, will convert the light to circularly polarized, but this makes no difference to your eye.

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    $\begingroup$ To add to this excellent answer: It's hard to manufacture waveplates that produce exactly the right phase delay for all visible wavelength and some deviations will occur at the far ends of the spectrum. In your case the waveplate is not a perfect quarter waveplate for blue light and will produce slightly elliptically polarized light for the linearly polarized blue light from the screen. So depending on the orientation of the glasses (and their integrated polarizer) they will transmit more blue light (blue tint) or less (yellow tint), while it's mainly constant for all other wavelengths. $\endgroup$
    – Emil
    Commented May 7, 2015 at 14:32
  • $\begingroup$ Worth emphasizing (you mention it in passing) that LCD displays typically produce linearly polarized light (and maybe explaining why that is so). As such, you will block the light if you put a polarizer at right angles - which is why you get extinction when you put the glasses in front with the linear polarizer facing the display. I think this answer can be improved with a couple of diagrams... $\endgroup$
    – Floris
    Commented May 7, 2015 at 15:12

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