here’s a big
debate on some physics forums as to whether the
three-polarizer phenomenon is proof of quantum mechanics or not.
Here’s the experiment.
Take three light polarizing filters. Polarized sunglasses will work. Cross two of them to block out the light. Then put the third one between them at a 45 degree angle. You will observe that the light is no longer blocked. An LCD TV is a good light source and can substitute for the first polarizer, as a backlit LCD TV emits polarized light.
Image of an LCD TV using two crossed polarizers at 45 degrees
Experimental tests
Linear polarizing filters don’t change the polarization state of light and they don’t create circularly polarized light. They merely block polarization in one direction. So how could this work? Let’s get some actual numbers.
A plastic polarization sheet will reduce light intensity by 16.8% due to reflection and absorption and reduce the remainder by 50% by polarization. The light can be easily measured using a photocell (taken from a broken landscape light), a voltmeter, and a light source. I also added a variety of filters to test the effect of wavelength.
With a 10-watt white LED lamp and a blue Wratten 47B filter, the 45° middle polarizer increased the transmitted light by 6.61×. No increase was seen with R72 (near-infrared) or H-alpha (656.3 nm) filters, indicating that the three-polarizer effect doesn’t work with near-infrared light. Ordinary halogen lamps don’t work either, evidently because they emit mostly in the infrared. A green (532 nm) semiconductor laser showed the biggest effect: a 46.68× increase (from 0.165 to 7.703 mV); however, the laser itself emits polarized light, which would have affected the result. A green LED was the best light source. Of course better polarization filters worked better, but at most only 25% of the incoming light was ever restored. For example, the signal from the green LED was reduced from 12.965 mV to 0.061 mV by two crossed polarizers, a loss of 99.53%. When the third 45° filter was added, it increased to 0.545 mV, an 8.93× increase but still only 4% of the original brightness. Correcting for reflection losses only increased this number to 7.3%.
How does this work?
What is going on? J. J. Sakurai, in Modern Quantum Mechanics, calls the three filters x (at 0°), x′ (which is at 45°), and y (which is at 90°) and notes that light is created at a polarization it didn’t have before:
[T]here is a light beam coming out of the y-filter despite the fact that right after the beam went through the x-filter it did not have any polarization component in the y-direction. . . . The selection of the x′-polarized beam by the second Polaroid destroys any previous information on light polarization.
Sakurai doesn’t seem to care about the effect other than to use it as an illustration of how polarization vectors are real and non-intuitive even for everyday phenomena. His interpretation is that the middle filter takes the polarized light, makes it ‘forget’ its original polarization, and adds its own new polarization state. How that could happen is a mystery. If the “destroying information” theory were true, then crossed polarizers wouldn’t block 99.53% of the light. The numbers also clearly show that it’s not just “crushing” the incoming light as SGOTI is saying.
Sakurai compares it to the famous 1922 Stern-Gerlach experiment, which showed that silver atoms possessed possessed a quantized spin state which allowed them to be separated into two beams by an inhomogeneous magnetic field. According to Sakurai, the abstract vector space that describes those spin states is analogous to the polarization vectors of the classical electromagnetic field.
There’s actually still a debate about what’s causing those silver atoms to move. Van der Straten says (p. 135) that spin has nothing to do with it and the S-G effect is solely due to magnetic moment. We know the magnetic field isn’t changing the rotation of the silver atoms because it is the spin of the single unpaired electron that provides the angular momentum (or magnetic moment as the case may be). This electron is not in any fixed location around the nucleus, so it would be impossible for it to cause alignment of the atom in a field. It can only cause movement toward or away from the magnetic field.
It’s remarkable that people are still arguing about the S-G experiment a hundred years later. No doubt they’ll be arguing about the three polarizer effect for just as long.
jun 23 2026, 7:40 am
