Three polarizing filters: a simple demo of a creepy quantum effect
Channel: Orion Lawlor
Date: 2019-04-25

Crossing two linearly polarizing light filters blocks the light.  But adding a third polarizing filter at a diagonal angle lets light through again, but only if the diagonal filter sits between the two crossed polarizers, not if it comes before or after them.

There's a good explanation of the physics here, where it's clear the filters actually interact with the photons instead of just passively reading their constant polarization.
http://alienryderflex.com/polarizer/

Because light is also made of photons, how exactly you can interpret what a filter does to a photon gets much less clear, and simple ideas like "A photon has a polarization" no longer match reality--the polarization you read depends on the filters you apply.  Paul Dirac used this exact experiment as an introductory example to quantum mechanical effects in his 1930 textbook "The Principles of Quantum Mechanics", excerpted here:   https://www.informationphilosopher.com/solutions/experiments/dirac_3-polarizers/

These are the linear polarizing filters I bought (back in 2015, hopefully still good!): 
   https://www.amazon.com/gp/product/B004X3XFHU/      
(Not an affiliate link, and I have no connection to this seller.)

I designed and 3D printed the red frames, and put the design files up here: 
   https://www.thingiverse.com/thing:4967649