Three polarizing filters: a simple demo of a creepy quantum effect

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.

These are the linear polarizing filters I bought (back in 2015, hopefully still good!):
(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:

[Thesignalpath]

EDIT: As Orion has pointed below, this effect can indeed be explained through Quantum Mechanics. It just happens that EM wave theory can also explain this effect if we describe it using linear superposition of the polarization angles. However, while the EM version "makes sense" the ultimate complete explanation is quantum mechanical in nature. Thank you Orion for pointing this out.

[magnificentmuttley]

That was really interesting. Thank you Orion. This is a perfect example of why YouTube was conceived in the first place, for interesting, educational short films like this. Brilliant.

[RudahReis]

It's easy to understand if you think the lens are "turning / twisting" the waves to the arrow direction. So when you put the third filter diagonally between the 1st and 2nd filter, you are just 'undoing' part of the turning the 1st filter did.

[gapspt]

This is a simple phenomenon and easily calculated with same math for doing vector decomposition/projection.
Imagine you have an ice cube that is inside some rails that only allow it to slide from left to right, and another one in another rail that only allows it to slide back and forth (i.e. perpendicular to the first rail). If you throw one against the other, the other will not move, because the impact force is perpendicular to the direction it is able to move. But if you have a third cube in a diagonal rail, you can push the first one against the diagonal which will then slide with ~0.7 of the speed of the first one, and that one can hit the third one which in turn will slide with 0.5 of the original speed, thus able to "rotate" a force by 90º (but losing some of the power), where before it seemed impossible. This will not work if that diagonal one is the first or the third, same as we see in the video.

[gharren]

The paradox appears by confusing the function of a polarizer with that of a filter. Polarizers do not strictly filter (remove) components of the light, but can _add_ a polarized component to the light. If completely vertically polarized light hits the horizontal polarizer or vice versa, it is completely filtered. However, as it passes through the 45-degree filter, some of the light will be both horizontally and vertically polarized, making it survive the final filter. This perfectly explains why less light is filtered when the angled polarizer is placed in the middle, rather than the beginning or the end. You do not require quantum mechanics to explain this effect, so by Occam's Razor, it is the most likely explanation.

[MateusSFigueiredo]

I searched "three polarized filters" and this was the first result on Google. I was not disappointed.

[jorgepeterbarton]

An intuitive thought to make it less disturbing:
A pipe you send balls down, theyd get stuck on a 90 degree turn (assumed). If you add a 45 degree on that turn (so 2 45 degree) then their path is easily directed around the corner. If you place 45 degrees before or after in such a way as the 90 degrees is unaffcted they halt.

So it doesnt seem that counter intuitive, although they are not pipes and this does not explain the mechanics at work....disturbing? Doesnt seem so, not like a delayed choice quantum eraser or anything, what would be disturbing is if the order didnt matter!

[odal]

Let us assume, for the sake of argument, that, instead of polaroid filters, we are using black filters with a hole in the center, We have to put our eye close against the hole to be able to see directly beyond it. Rotating the first filter misaligns the hole relative to the hole of the second filter, showing us, besides the scene directly behind the first filter, the black and opaque surface of the second filter.
If we now put a third filter between the first and the second, at 45 degrees, still with our eye against the first hole, we are able to look through the hole of the third filter, at least partially, all the way through the second filter and beyond.

[hamdaniyusuf_dani]

The experiment of polarization is best done using microwave, since you can see the physical structure of the polarizer. In case of three polarizers, we can demonstrate that the light coming out from the diagonal polarizer has diagonal polarization state. By turning the third polarizer in line with the second polarizer, you'll get brighter light compared to when it's vertical or horizontal.
I've made many videos investigating behavior of microwave transceiver here in Youtube. I conclude that objects interact with electromagnetic wave following the same rules as how radio antennae work.

[PeterMancini]

For me the best way to look at is, we assume light is particles. Which it is sometimes. But it can also be "waves" and those waves simply are just there is a chance there is a particle here with a certain orientation, or it over there in another orientation, or its in all locations and all orientations and you won't know until it is observed. So, what happens is there is a chance the light passes through the first two, then we do it again for the second two and then you observe it and is goes from decoherence of being a set of probabilities of being here or there in this or that orientation to into coherence because you observe them. It does not work with laser light. Its the same principle of the X-RAY specs sold at the back of comics in the old days that let you look under the clothes of people. :-) The Men in Black wear these shades for exactly that reason.

[danielnsb]

Really deep and interesting stuff!

Do you have a link for the filters you use? (Just find some filters without the practical plastic frame)

[colinbutts]

The 'Three Polarizing Filters' experiment is the BEST demo of quantum physics.
I would enjoy hearing anyone explain how this works; where the hoi polloi (me) could understand.
The first principle is that you must not fool yourself and you are the easiest person to fool. - R. Feynman

[kilroy]

It's more like telling the photons come through the first filter to polarize diagonally, which isn't enough for the third filter to block them.
The other two filters combined with no interruption will always block all light.
So that makes sense to me.

[manla]

I thought you can explain this phenomenon by the wave nature of light.

[feelthewyrd]

i think there is a common sense answer that denies the unique 'strangeness' of the QM explanation. If we replace 'filter' with 'how easy is it to twist' then instead of

What percentage of light is 'filtered' by a polariser at angle x, we ask 'what percentage of light is twisted by a polariser at angle x'.

To twist light 90 degrees is not possible with one polariser, because it is impossible to twist light 90 degrees with one polariser.
But it is possible to twist light 90 degrees with 2 polarisers, because any angle less than 90 degrees is possible to twist with one polariser. So long as our 2 polarisers have a combined twisting of 90 degrees, but each is less than 90 degrees .... then no surprise.

Its like saying "If i cant twist a screwdriver 180 degrees with one twist, then i can break it down to 2 twists". The most efficient would be a half way to the desired twist angle for both twists... ie 90 degrees for the hand held screw driver.

So similarly polarisers that are not capable of twisting light 90 degrees in one go .... then use 2 polarisers instead .... with each twisted at 45 degrees, to be the most efficient light twisting of 90 degrees using 2 polarisers. (2 equally difficult twists)

looked at this way there is no mystery at all. We use screwdrivers all the time and and we dont grab hold of the handle and run around in circles. We break it down into multiple twists. But the reason we are tricked into thinking the polariser filter paradox is strange is because we think of filtering ... not twisting. If they were called 'polarising twisters' we wouldnt be surprised by the effect at all.

[helmutalexanderrubiowilson]

quantum efect; 1) first filter vertical: only vertical polarized photons pass 2) filter diagonal: 50% of diagonal polarization pass 3) filter horizontal: 50% of horizontal polarization pass from diagonal polarized photons.

[dang]

My mind says it’s just bending the light between the horizontal and vertical to be in between, diagonal, but once you block the light with sequential filters the light has been stopped already.

[michaelvrijhoef]

The "quantum" part is not to see light as only photons. QED has taught us that they are both wave and particle, or neither. Quantum field theory, Feynman,
Best way to think about it is in a probabilistic wave approach, rather than a deterministic. Which then explains the "why" behind the questions asked. Their probability (i.e. the probability of light/photons passing through) concurs with light intensity passing through as shown in these experiments.

[imnewtothistuff]

Why this always confuses Simple-Minded people Is because some commenters think light is a particle and some commentors think light is a wave. well, it's neither, Light is a coaxial perturbation of the aether. It is neither a particle nor a wave. Light does not travel. It propagates, there is nothing moving from here to there. Nikolai Tesla said "Light propagates through the aether the same way sound propagates through air." The speed of the action is dependent on the medium that it's propagating through. Light propagates at 186, 284 mps. That is the rate of induction of the aether. Sound propagates roughly 1100 feet per second in air, at sea level. Through water, sound propagates many times faster, through steel, sound propagates at 85, 000 ft per second. So you see, it's all dependent on the medium. Light and sound are nothing more than compression rarafaction events of a medium, Frequency specific.
You're welcome...
.

[customsongmaker]

No, you're not asking the same question each time. In 3 of the arrangements, you're asking "Can I see horizontally-filtered light through the vertical filter?"

By putting the diagonal filter in between, you're changing the horizontal light to diagonal light. Then the question becomes, "Can I see diagonal light through the vertical filter?"