Polarization of Light
Light from a Red Diode Laser (peak wavelength = 650 nm) is passed through two linear
polarizers. As the second polarizer (the analyzer) is rotated by hand, the relative light intensity is
recorded as a function of the angle between the axes of polarization of the two polarizers. The
angle is obtained using a Rotary Motion Sensor that is coupled to the polarizer with a drive belt.
A polarizer only allows light which is vibrating in a particular plane to pass through it. This
plane forms the "axis" of polarization. Unpolarized light vibrates in all planes perpendicular to
the direction of propagation. If unpolarized light is incident upon an "ideal" polarizer, only half
of the light intensity will be transmitted through the polarizer.
Figure 1: Light Transmittedthrough TwoPolarizers The transmitted light is polarized in one plane. If this polarized light is incident upon a second
polarizer, the axis of which is oriented such that it is perpendicular to the plane of polarization of
the incident light, no light will be transmitted through the second polarizer. See Fig.1.
However, if the second polarizer is oriented at an angle not perpendicular to the axis of the first
polarizer, there will be some component of the electric field of the polarized light that lies in the
same direction as the axis of the second polarizer, and thus some light will be transmitted
through the second polarizer.
Figure 2: Componentofthe Electric Field
If the polarized electric field is called E 0fter it passes through the first polarizer, the component,
E, after the field passes through the second polarizer which is at an angle with respect to the
first polarizer is E cos (see Fig.2). Since the intensity of the light varies as the square of the
electric field, the light intensity transmitted through the second filter is given by
I I cos 2 (1)
o THEORY FOR 3 POLARIZERS
Figure 3: Electric Field Transmitted throughThree Polarizers
Unpolarized light passes through 3 polarizers (see Fig.3). The first and last polarizers are
oriented at 90 with respect each other. The second polarizer has its polarization axis rotated an
angle from the first polarizer. Therefore, the third polarizer is rotated an angle from
the second polarizer. The intensity after passing through the first polarizer is I a1d the intensity
after passing through the second polarizer, I ,2is given by
I2 I1cos .
The intensity after the third polarizer, I3, is given by
I3 I2cos 2 I co1 cos 2 (2)
It is possible to simplify this expression to the following:
I 1 sin (2) (3)
Note: This means that when there are two perpendicular polarizers with a third rotatable
polarizer between them, the maximum intensity through all three polarizers is one quarter of the
maximum intensity through two polarizers (one of which is rotatable), or:
4 4 SET UP
Note: most or all of the setup is already done for you. This section is provided for reference.
Figure 4: Equipment Separated to Show Components
1. Mount the aperture disk on the aperture bracket holder.
2. Mount the Light Sensor on the Aperture Bracket and plug the Light Sensor into the
interface (See Fig.5).
Figure 5: ScienceWorkshop 500 Interfacewith Sensors
3. Rotate the aperture disk so the translucent mask covers the opening to the light sensor
Figure 6: Use Translucent Mask 4. Mount the Rotary Motion Sensor on the polarizer bracket. Connect the large pulley on
the Rotary Motion Sensor to the polarizer pulley with the plastic b