What are effective methods of polarization?

1 Answer
Dec 19, 2014

There are various methods to polarize a wave and, according to the wavelength, you'll use some of them.
For example, microwaves can be polarized using a metal grate similar to the one used to cook meat in a barbecue.
To polarize visible radiation you can use crystals like calcite or plastic filters called polaroids.
A very easy and practical way to polarize visible radiation (from the Sun for example) is through REFLECTION.
enter image source here
( Source: Wikipedia)
During reflection, radiation emerges polarized according to the incidence angle. Maximum of polarization occurs when light forms a special angle of incidence with the surface, the Brewster's Angle.

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As you can see in the picture the Brewster's Angle, B, is obtained when the angle between reflected, r, and refracted, R is equal to 90°.
When you get this condition the reflected ray has maximum polarization.
But how do you evaluate this angle?
You use Snell's Law at the interface between the two media, 1 and 2.
#n_1sin(theta_1)=n_2sin(theta_2)#
but #theta_2# the refracted angle can be written as: #90-theta_1#
substituting in Snell's Law you get:
#n_1sin(theta_1)=n_2sin(90°-theta_1)=n_2cos(theta_1)#
enter image source here
Rearranging and passing to inverse trigonometric function you get:
#theta_1=arctan(n_2/n_1)# which is the Brewster's Angle.

You can see an example of this phenomenon when you look at light reflected by the windows of a car under the Sun in a parking . That is polarized light! If you try to see through a polaroid you'll be able to see the interior of the car!!!