In “The Electric Boy” episode of “Cosmos”, Faraday is shown investigating if magnetism affects light. Here’s a screen shot of the experiment. In the center is an electromagnet with a beam of polarized light going over the top. Faraday is looking through a polarized lens. In the initial state of the experiment, the light does not pass through the lens because it has the wrong polarization.
Faraday turned on the magnet, but the polarization did not change. He tried passing the light through various materials and solutions, but most of them did nothing. Eventually, he passed the light through a block of glass on top of the magnet and that changed the polarization and he could finally see the light through the lens. I am confused about a few of things.
Why did Faraday start the experiment with the light not passing through the polarized lens? Why not start the experiment where he could initially see the light and that any change to polarization would block the light? It seems that if light was initially passing through the lens, it would be easier to notice if there were any changes to the polarization because the light would become dimmer or totally blocked.
How did the glass block change the polarization of light with the magnet?
How did Faraday know that the magnet affected the beam of light and not the atoms in the glass? That is, maybe the magnet affected the atoms, caused them to move, and that the new crystal structure in the glass caused the polarity to change.
It’s a lot easier to tell the difference between a little bit of light and no light at all than it is to tell the difference between a lot of light and a little bit less light. This is true even if you’re using electronic instruments, but especially true if your only light-detecting instrument is the human eyeball.
As I understand it the magnet induces a movement of electrons in the medium of propagation. That movement creates other magnetic fields. The field is stronger in one direction than the other. Linearly polarized light is the superposition of equal amplitude RHC and LHC fields. The induced field in the medium “slows” one of the two circular components, causing a phase shift in the output linear polarization.
The magnet did affect the atoms in the glass. The size of the effect is the Verdet constant, which is dependent on temperature, wavelength, and the material itself. It would work in air as well, but the constant is so small that Faraday could never have measured it. It looks like in 2011 somebody did make the measurement and got 1.39×10(-9) rad G(-1) cm(-1) at 634.8 nm.