Polarization By Scattering and Reflecting

Unpolarized light can be polarized artificially, as well as by natural phenomenon like reflection and scattering.

Learning Objective

Calculate angle of reflection of complete polarization from indices of refraction

Key Points

When unpolarized light hits a reflective surface, the vertically polarized aspects are refracted into the surface. The horizontally polarized aspects are reflected off the surface and the light is now perceived as partially polarized.
When light is reflected, there is an angle at which this light is completely polarized. This is called Brewster's angle, after the Scottish physicst who discovered the law.
Unpolarized light can also become polarized when it is scattered in air (also known as Rayleigh scattering). This occurs due to the fact that the EM waves cause the electron in air to vibrate, producing radiation and causing polarization of the light.

Terms

index of refraction
For a material, the ratio of the speed of light in vacuum to that in the material.
polarization
The production of polarized light; the direction in which the electric field of an electromagnetic wave points.

Example

At what angle will light traveling in air be completely polarized horizontally when reflected from water? First, lets remember the index of refraction of air and water:nair - 1.00nwater- 1.33 Now, we can apply Brewster's Equation and solve for θ_b: $tan heta_b= rac{n_water}{n_air}\tan heta_b= rac{1.33}{1.00}\tan heta_b=1.33\ heta_b=tan^{-1}1.33\ heta b=53.1^{ rc}$

Full Text

Polarization by Reflection

In the previous atom we discussed how polarized lenses work. In the case of polarized sunglasses, for example, when you look through them, reflected light is not entirely filtered out; reflected light can be slightly polarized by the reflection process (as shown in ). Most light sources produce unpolarized light. When light hits a reflective surface, the vertically polarized aspects of that light are refracted at that surface. The reflected light is more horizontally polarized. To better remember this, we can think of light as an arrow and the reflective surface as a target. If the arrow hits the target perpendicularly (vertically polarized), it is going to stick in the target (be refracted into the surface). If the arrow hits the target on its side (horizontally polarized) then it will bounce right off (be reflected).

Polarization by Reflection

Unpolarized light has equal amounts of vertical and horizontal polarization. After interaction with a surface, the vertical components are preferentially absorbed or refracted, leaving the reflected light more horizontally polarized. This is akin to arrows striking on their sides bouncing off, whereas arrows striking on their tips go into the surface.

Since the light is split into two, and part of it is refracted, the amount of polarization to the reflected light depends on the index of refraction of the reflective surface. We can use the following equation to determine the angle of reflection at which light will be completely polarized:

$tan \theta_b=\frac{n_2}{n_1}$

where: θ_b = angle of reflection of complete polarization (also known as Brewster's angle); n₁ = index of refraction of medium in which reflected light will travel; and n₂ = index of refraction of medium by which light is reflected.

Polarization by Scattering

Just as unpolarized light can be partially polarized by reflecting, it can also be polarized by scattering (also known as Rayleigh scattering; illustrated in ). Since light waves are electromagnetic (EM) waves (and EM waves are transverse waves) they will vibrate the electrons of air molecules perpendicular to the direction in which they are traveling. The electrons then produce radiation (acting like small antennae) that is polarized perpendicular to the direction of the ray. The light parallel to the original ray has no polarization. The light perpendicular to the original ray is completely polarized. In all other directions, the light scattered by air will be partially polarized.

Polarization by Scattering

Also known as Rayleigh scattering. Unpolarized light scattering from air molecules shakes their electrons perpendicular to the direction of the original ray. The scattered light therefore has a polarization perpendicular to the original direction and none parallel to the original direction.

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