Optical Bench
Polarization and Brewster’s Angle
When light is reflected from a surface it can become polarized. If light is reflected at a sharp
angle from a surface (such as light from a sunset reflecting from the surface of a lake, the
polarization can be dramatic. This is why polarizing sunglasses block horizontally polarized light
in order to reduce sun glare. The angle at which polarization is maximized is called Brewster’s
angle.
Equipment:
Optical Bench
Cautions:
This equipment is delicate. Everything should go together with the lightest of touches. Do not
force anything!
Procedure to set up the Optical Bench
1. Place the optical bench on the table with the zero point of the scale to the left.
2. Place Light Source with filament (vertical notch reference along bottom edge) at 20
cm on the Optics Bench shining toward the right end of the bench.
3. Find one of the taller component holders with a solid flat foot
4. Mount the Slit Plate on the “foot” side of a Component Holder. The foot will point
toward the light source.
5. Mount the Slit Mask on the other side of the same Component Holder (taller frame
with solid flat foot).
6. Place the Component Holder so that its vertical plate is at the 30 cm mark on the
Optics Bench and its foot points toward the light source.
7. Attach the round Ray Table to the Ray Table base with the protractor side (not the
“grid” side up.
8. Mount the Ray Table and Base to the optical bench to the right of the component
holder so that the center of the Ray Table is at the 40 cm mark on the bench and the
“downhill” edge of the Ray Table is closest to the Light Source.
9. Turn the round ray table so that the zero degree Normal Line is parallel to the length
of the Optics Bench.
10. Connect the Light Source power supply and turn the Light Source on.
11. Align the Slit Plate and Slit Mask so that light from just one slit shines along the
center “NORMAL” line of the Ray Table. This alignment requires that one slit of the
Slit Plate be centered in the Light Source beam, and that the Slit Mask opening is
centered on that one Slit Plate slit, and that the Ray Table is turned to align the
Normal Line with the light ray. One other adjustment that may be necessary is some
slight rotation of the knob on top of the Light Source, which rotates the lamp and
filament inside to fine tune the position of the light ray.
12. Mount one Polarizer on a solid-foot Component Holder with its zero degree mark at
the top,
13. Position the holder on the Optics Bench between the Light Source and the Slit Plate.
14. Mount the second Polarizer on the Ray Table Component Holder (the shorter frame
with diamond hole in foot); on the side of the Holder frame away from the foot. This
Polarizer must also be oriented with zero degrees at the top.
15. Position the second Polarizer and Holder on the edge of the Ray Table farthest from
the Light Source.
Now you will observe the effect of passing light through two polarizers at different
angles.
1. Look through the polarizer mounted on the ray table back toward the light source
(you may have to move your head back and forth a bit to find the spot at which the
light passes through all the assembled components. You are looking for the image of
the filament itself. Observe the brightness of the filament image. Don’t look at the
filament for too long to avoid afterimages.
Note: you can discern 6 levels of relative brightness (bright, dim, medium, medium bright,
medium dim, or no light at all). Please use these terms while recording your observations.
Write your observation here (hint: this one is bright!)
Relative Brightness at 0°:
2.
Leave the second Polarizer in place on the Ray Table, but reorient the Polarizer by
rotating it counterclockwise so the 30 degree mark is at the top. Continue in 30
degree steps up to and including 180 degrees orientation, and observe the brightness
of light transmitted through the polarizers set at each angle and record the relative
brightness of light below:
Relative Brightness at 30°:
Relative Brightness at 60°:
Relative Brightness at 90°:
Relative Brightness at 120°:
Relative Brightness at 150°:
Relative Brightness at 180°:
Now you have some experience with determining the brightness of the transmitted
light. You will use this experience in the next procedure
3.
Remove the first Polarizer and Holder from its place between the Light Source and
Slit Plate. Put this Polarizer and Holder away carefully.
Next, you will observe the effect on polarization of light passing through a lens.
1.
Place the Cylindrical Lens on the Ray Table with the label “Cylindrical Lens” up
and the curved face away from the Light Source. Adjust the Cylindrical Lens so that
the flat surface facing the Light Source is centered and aligned on the “Component”
line of the Ray Table so that the light beam remains aligned with the normal line.
2.
Observe the brightness of the light ray transmitted through the lens with the Polarizer
oriented at zero degrees, again at 45 degrees, and once more at 90 degrees, and
record your observations here: Make sure that you look through the bottom half of
the polarizer so you are seeing light that passes through the lens.
Relative Brightness at 0°:
Relative Brightness at 45°:
Relative Brightness at 90°:
3.
Rotate the Ray Table 70 degrees clockwise (light ray falls along the 70 degree line).
Move the Component holder with the polarizer 40 degrees counterclockwise around
the edge of the Ray Table to position it in line with the transmitted ray.
4. Observe the brightness of the light ray transmitted through the lens with the Polarizer
oriented at zero degrees, again at 45 degrees, and once more at 90 degrees, and
record your observations here:
Relative Brightness at 0°:
Relative Brightness at 45°:
Relative Brightness at 90°:
Now you will observe the effect on polarization when a beam of light is reflected from a
surface.
1.
Move the ray table counterclockwise 140 degrees so that the incoming ray sits on the
other 70 degree mark on the Ray Table
2. Align the polarizer in front of the reflected ray from the cylindrical lens. It should be
somewhere near the other 70 degree mark on the table.
3. Observe the intensity with the Polarizer oriented at zero degrees, again at 45 degrees,
and once more at 90 degrees, and record your observations here:
Relative Brightness at 0°:
Relative Brightness at 45°:
Relative Brightness at 90°:
4.
5.
6.
Rotate the Ray Table 10 degrees clockwise (light ray falls along the 60 degree line).
Move the Polarizer 10 degrees clockwise (to the 60 degree position).
Observe the intensity with the Polarizer oriented at zero degrees, again at 45 degrees,
and once more at 90 degrees, and record your observations here:
Relative Brightness at 0°:
Relative Brightness at 45°:
Relative Brightness at 90°:
7.
8.
Rotate the Ray Table 10 degrees clockwise (light ray falls along the 50 degree line).
Move the Polarizer 10 degrees clockwise (to the 50 degree position).
9.
Observe the intensity with the Polarizer oriented at zero degrees, again at 45 degrees,
and once more at 90 degrees, and record your observations here:
Relative Brightness at 0°:
Relative Brightness at 45°:
Relative Brightness at 90°:
10. Rotate the Ray Table 10 degrees clockwise (light ray falls along the 40 degree line).
11. Move the Polarizer 10 degrees clockwise (to the 40 degree position).
12. Observe the intensity with the Polarizer oriented at zero degrees, again at 45 degrees,
and once more at 90 degrees, and record your observations here:
Relative Brightness at 0°:
Relative Brightness at 45°:
Relative Brightness at 90°:
13. Rotate the Ray Table 10 degrees clockwise (light ray falls along the 30 degree line).
14. Move the Polarizer 10 degrees clockwise (to the 30 degree position).
15. Observe the intensity with the Polarizer oriented at zero degrees, again at 45 degrees,
and once more at 90 degrees, and record your observations here:
Relative Brightness at 0°:
Relative Brightness at 45°:
Relative Brightness at 90°:
16. Turn off the Light Source and put the materials away carefully, unless the Teacher
instructs you to leave specific items in place for the next class.
Data Analysis:
You will now examine the data collected to see if there are any trends. The first set of
data with two polarizers should be fairly straightforward. Answer the following
questions after referring to the data taken for two polarizers:
At what polarizer angle(s) was the light brightest?
At what polarizer angle(s) was the light least bright?
Next, you’ll examine the data collected when the light passed through and was refracted
by the cylindrical lens. Look at your data and answer the following question:
Did the angle of light transmission affect the brightness of the image at any angle of
refraction or polarization?
Finally, you will look at the relationship between polarization and angle of reflection
You have a lot of data for this, so you’ll tabulate it below. You might find it to be more
convenient to use numbers to represent the observed brightness of the filament.
0=dark, 1=dim, 2=medium dim, 3=medium, 4=medium bright, 5=bright
Incident Angle
70°
60°
50°
40°
30°
0°
Polarizer
Angle
45°
90°
The definition of “Brewster’s Angle” is the angle of reflection at which the light is
completely polarized parallel to the reflecting surface. At angles less than or greater than
Brewster’s angle, the light is only partially polarized.
Examining the data in the table above, make an estimate of Brewster’s angle for the
reflections. It is the incident angle at which the difference between light intensity at 0
degrees and 90 degrees polarizer angle is greatest. Discuss your estimate below: