## Faraday’s Electromagnetic Wave Experiment: How One Experiment Defined Light

I was taught in high school that light is an electromagnetic wave.  But what does that mean and how in the world was it discovered?  This is a presumptuous college student, mysterious crystals, what polarizers are, and how polarizing sunglasses work.  Ready?  Let’s go.

Fast-forward 17 years to 1845.  Michael Faraday was 54 years old and was already famous for his discovery of the first motor, the ideas of magnetic fields, and induction as well as thousands of other discoveries.  He also suffered from depression and memory problems (as well as possibly mercury poisoning, a common affliction for scientists at that time as they didn’t know the dangers of mercury) and was spending very little time in the laboratory.  This is when a baby-faced 21-year-old mathematics student named William Thompson(later knighted Lord Kelvin) presumptuously asked Faraday if he had ever attempted to change something called the polarization of light with magnets or electricity while the light was moving through a transparent material.  Faraday was intrigued, ever since Summerville’s disastrous experiment Faraday had been trying (and failing) to alter magnetism with visible light or to alter light with magnetism.  He had even tried to change the polarization of light, but never while the light was in a material.  Inspired, Faraday went back into the lab.

Now, what is polarization?  Imagine you are making a wave on a long rope.  To make the wave you could move your hand up and down and the wave would move away from you down the rope.  However, if you wish, you could also move your hand side to side.  This direction of vibration is called the polarization of the wave.  Now, most light is what is called unpolarized or randomly polarized, meaning that some is vibrating up and down, some side to side, and even some in clockwise and counterclockwise circles.

Now we come to a strange object called an “Iceland crystal”.  When the light goes from air into a transparent material the light slows down and bends.  For prisms, the amount the light bends is strongly dependent on the frequency of the light (the color), which is why prisms will split white light into a rainbow.  Icelandic spar is unusual because the amount the light bends is dependent on the polarization of the light, not on the frequency.  Therefore, when white light enters the crystal, it bends the light into two beams: one that vibrates vertically and one that vibrates horizontally.  As humans can’t distinguish polarization with their naked eyes, the crystal makes two images.

In 1818, a man named David Brewster published an article that if light reflects off of a surface at a low angle (currently called Brewster’s angle) then all of the reflected light will be polarized parallel to the surface.  If you look at this light with a properly oriented Iceland crystal you only got one image.  This is why polarizing sunglasses work so well, they block the horizontally polarized light, which blocks the light reflected at low angles, also called glare.

Ten years later a Scottish scientist named William Nicol invented a device that was the last piece of the puzzle.  Nicol wanted to make the Iceland crystal not just split the light into two polarizations, but actually, remove one polarization entirely – or make the first polarizing filter.  He did this by cutting two crystals into triangles at very specific angles and then gluing them together.  When light hits the crystal, the light splits into horizontally and vertically polarized light.  However, the horizontally polarized light is bent more so that when it hits the glue all of the light is reflected (this is called total internal reflection) and shines out the side of the prism.  The vertically polarized light, however, bends at the glue and then bends back into the glass on the other side of the glue and thus passes safely through the prism.

Now we have all of the pieces of Faraday’s experiment.  Faraday shined light from the brightest lamp he could get at Brewster’s angle off of a piece of glass (getting horizontally polarized light).  He then put a Nicol prism in the path of the light so that it filtered out the horizontally polarized light and he could no longer see the light from the lamp.  Faraday then placed a strong electromagnet (magnet made with electricity) next to a sample of glass in the path of the light.  When he magnetized the electromagnet he could see the light from the lamp!  The magnetic field had rotated the polarization of the light!  When he unplugged the electromagnet the image disappeared again.  Faraday said that he had “established, I think for the first time, a true, direct relation and dependence between light and the magnetic and electric forces.”

Just as with Summersville, the scientific community was excited to hear that light and electricity, and magnetism was linked in, “one great universal principle”.