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# Why Don't 3D Glasses Work Backwards?

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It's a question that has occurred to anyone who has ever sat in a theater fiddling with their theater-provided 3D shades waiting for the promos to finish rolling up: Why don't these work if I flip them around? Here's the physics behind the persnicketiness of 3D glasses.

In this thread, commenter (and physicist) FlowerGirlPhysicist explained just what was happening when you flip your glasses from back to front:

neisseria

Why do 3D glasses only work in one direction? IE why can't you turn the glasses around and still use them?

FlowerGirlPhysicist

Okay! So they make 3D glasses out of a stacked quarter wave plate and a linear polarizer.

The quarter wave plate takes linearly polarized light (from your LCD) and makes it left or right circularly polarized (movie theaters actually emit circularly polarized light, actually). Circularly polarized light has equal parts vertical and horiztonally polarized light. The linear polarizer (in the vertical direction) cuts out all the horizontal light.

Here's how it works forward:

If your LCD puts out vertically polarized light (VP):

• VP -> quarter wave plate -> Circularly polarized (CP) -> vertical polarizer -> 1/2 VP
• So in this case you're missing half your light, as you should.

If your LCD puts out horizontally polarized light (HP):

• HP -> quarter wave plate -> Circularly polarized (CP) -> vertical polarizer -> 1/2 VP
• You still get the same result.

Run it backwards though...

If your LCD puts out vertically polarized light (VP):

• VP -> vertical polarizer -> 1 VP -> quarter wave plate -> CP
• Now you get all your light back, you're not missing the half that you should

If your LCD puts out horizontally polarized light (HP):

• HP -> vertical polarizer -> 0! -> quarter wave plate-> still 0
• Now you get nothing. Black glasses.

So, there you have it

"It" in this case being both an answer to the question, and this GIF of a cat wearing 3D glasses that accompanied the question.