Science Forums 2

[CraigD]

Originally Posted by Little Bang

Since EMR has both electric and magnetic components how do we know that a magnet field doesn't cause something like rotation of the wave?

We know that if a medium containing electrons (eg: glass or interstellar gas) is subjected to a magnetic field, polarized light that interacts with that medium (ie: passes through it) has its plane of polarization rotated. This is the Faraday effect. It depends on the wavelength of the light, the density and molecular structure of the medium, and the orientation and strength, but not the direction, of the magnetic field. Different materials can rotate the plane in a positive or negative direction, and if the magnetic field is oriented perpendicular to the light beam, zero rotation occurs.

According to theory, as the density of electrons in the medium is reduced, so is the rotation. Visible light in one of the most effective Faraday rotator mediums, terbium gallium garnet, has a Verdet constant of -40 rad/T/m, glass around 1.8e-7, typical interstellar gas about 2e-20. (from data and links from the preceding wikipedia link, and some simple calculations)

Originally Posted by Little Bang

If there have been experiments that confirm magnetic fields have no effect what so ever on light please point them out.

This “proving a negative” would be pretty tough. The best experiments can show, I think, is that as a medium approaches a perfect vacuum, its Verdet constant approaches zero. To measure a Verdet constant of absolute zero, you’d need to have a vacuum better than found in deep space stretching for many light years and subjected to a strong magnetic field, and even then, you’d not be sure that your experiment was not sufficiently sensitive to detect some tiny effect not predicted by theory.

Most measurements of very small Verdet constants (usually called “rotation measure”, and with an additional unit to allow multiplication of the square of the light’s wavelength) are made in combination with other measurements to determine the density of matter in interstellar space. Given that theory predicts a zero Verdet constant for absolute vacuum, and all experimental data supports it, I doubt many experimental resources will be spent attempting to support the theory greater certainty.

Originally Posted by Little Bang

So if we pass a beam of light through a moving magnetic field experiment has shown that the light is not polarized?

It’s important to note that the Faraday effect doesn’t itself polarize light, but rotates the plane of polarization of previously polarized light. Un-polarized (randomly polarized) light will still be randomly polarized after its many planes are rotated by the Faraday effect.

The Faraday effect doesn’t require a changing/moving magnetic field. Change in the strength and orientation of the field is averaged to determine the total rotation of the material.