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CONCEPT ENHANCER - Polarized Light
Light as an Electromagnetic Wave
Before you can understand why some light is polarized and other light is not you need to
learn a bit about light waves. Light is an electromagnetic wave. You may have heard
about the electric field surrounding an electric charge and magnetic fields around
magnets. Light is not an electric charge and it is not a magnet, so what do
electromagnetic waves have to do with light? An electric charge has an electric field all
around it. If you take this electric charge and vibrate it, it will create a vibrating magnetic
field. Since you are vibrating the electric charge, both the magnetic and electric fields
also vibrate. When the electric charge vibrates up and down the electric field also
vibrates up and down and the magnetic field vibrates side to side. These two fields travel
together through space as an electromagnetic wave. If the frequency is just right we can
see this electromagnetic wave as light. If the charge vibrates too rapidly or too slowly it
still creates an electromagnetic wave, but it may be infrared or ultraviolet light or some
other form of electromagnetic wave that we cannot see.
When the electric field is vibrating in one dimension (vertical) we say that the light is
vertically polarized. In most materials the many little charges can vibrate in any direction
so they produce electromagnetic fields that are vibrating in many different directions.
This is unpolarized light. If your instructor hasn’t already demonstrated this model of an
electromagnetic wave try the following. Let one hand represent the electric field
vibrating up and down by standing up and swinging your arm up and down. Let your
other hand represent the magnetic field vibrating side to side by swinging your other arm
left and right (this takes a little practice and coordination). Now, when you walk forward
with your arms swinging you are like a light wave that is traveling through space with its
electric field vertically polarized and its magnetic field horizontal as you walk forward.
Polarized Light
Now that you understand a little about how light acts as an electromagnetic wave you are
ready to take a look at how things get polarized. Polarized filters are designed so that
they allow light with an electric field vibrating in one direction to pass through
unaffected, but will block any light that has an electric field in the wrong direction. It is
kind of like carrying a meter stick through a narrow doorway. If your meter stick is
vertical, like the doorway, then you can pass through it. But, if you hold your meter stick
horizontal, you will not be able to get through the doorway. In Figure 12.1, if the
polarizer filter is aligned to pass vertical light all of the light in part A will pass through
unaffected, but all of the light in B will get blocked. But what will happen to the light in
part C? In part C the polarizer will only pass the component of the electric field that is
parallel to the polarizer axis (remember vector components).
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Polarizer filter
Vertically
polarized
light
Polarizer filter
Horizontally
polarized
light
A
B
Polarizer filter
Diagonally
polarized
light
C
Figure 12.1
In the activities SHADY BUSINESS and LET’S REFLECT ON THAT you probably
noticed that some light that is reflected is polarized, even when it wasn’t polarized before
the reflection. This is because light that hits a horizontal surface interacts differently if its
electric field is vertically polarized or horizontally polarized. Since most light is
unpolarized it is a jumbled mix of both vertically, horizontally, and diagonally polarized
light. The light with horizontally polarized electric waves just bounce off the surface like
rocks skipping off a lake, while light with vertically polarized electric waves gets
absorbed by the material. So, horizontal surfaces have reflections that are mostly
horizontally polarized. Likewise, reflections off of vertical surfaces will be mostly
vertically polarized. Polarized sunglasses are usually made so that they block
horizontally polarized light. Can you think why this might be the case?
You may have noticed that not all reflections are polarized. Because electrons in a metal
are so free to move around and respond to electromagnetic waves they are much better at
reflecting both vertically polarized and horizontally polarized light, no matter what the
orientation of the metal surface. So reflections off of metal surfaces are usually not
polarized.
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Score ______
Period _____
Names _________________________
CONCEPTUAL PRACTICE
1. When the filament of a light bulb gets hot, the particles within the filament vibrate in all
directions. What polarization (vertical, horizontal, diagonal, or none) will this light
have?
2. Lasers take a single light wave and exactly duplicate it billions of times. Would you
expect this light to be polarized? Why or why not?
Keep going – turn over
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3. In the following diagrams the degree of polarization is depicted, followed by the
polarizing filter with its orientation and reflecting surfaces. Use arrows to represent the
polarization of the light after it is reflected or passes through the polarizer filter.
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