On Further Reflection

As we were driving into Boston today, Steve was marveling at the appearance of the clouds through his polarizing sunglasses.  I figured it was a good excuse to talk a bit about polarization, and why it's useful in sunglasses.

Light is made up of oscillating electric and magnetic fields (hence its alternate name, electromagnetic wave).  Polarization refers to the direction of the electric field, which can be fixed in one direction (linear polarization), or can rotate about the axis the light travels along (circular/elliptical polarization).  Typical light from the sun or other mundane sources is unpolarized, meaning it consists of a random assortment of polarizations.  However, under certain conditions, reflected light can become polarized.

When light hits a reflective surface at a specific angle (called Brewster's angle), the reflected light will be completely polarized parallel to the reflective surface.  For angles close to Brewster's angle, the reflected light will be mostly polarized in the same direction.  By cutting out that particular polarization, the reflected light will be blocked.  Here's a picture demonstrating the effect (from Wikipedia):

The photo on the left was taken without a polarizing filter, while the one on the right was taken with a filter adjusted to cut out the light reflected from the window.  This can be useful when driving by eliminating reflections from puddles on the road, or glare on glass.

Polarizing lenses work by having electrons that are free to move along a particular axis.  When the light hits the lens, if its electric field is aligned with the axis the electrons can move along, the field causes the electrons to start oscillating with the field.  This absorbs the light and re-emits it in the opposite direction, in essence reflecting it.  Therefore, any light emerging from the lens has polarization perpendicular to the direction the electrons move in.

In addition to its usefulness in driving, polarization can also create cool effects like the clouds I mentioned in the opening.  Here's a photo of a similar situation (from Wikipedia):