Next: Lab 7. Antenna Measurements
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Polarization, reflection and refraction of plane electromagnetic waves are described in
RWVD, Chapter 6 (SES Chapter 8). Please review RWVD Secs. 6.3, 6.9-6.13 or SES Chapter 8. In
this demonstration you will examine these phenomena for light waves, using a He-Ne laser
(Spectraphysics 155) as a source of electromagnetic waves (light) at wave-length 632.8 nm (red
light). [You can also examine the construction of HeNe laser (Electro Optics Associates
Las-101)].
You will study Snell's law, total internal reflection, and Brewster's angle, and their
polarization properties. The experiment is shown below:
- Examine the LAS-101 HeNe laser. Note the gas discharge tube, Brewster's angle
windows, and high reflectivity (for red light) mirrors. View the white light in the
room through the mirrors. What color do you see, and why?
- Turn on the Spectraphysics 155 laser. Direct the beam through the hole in the
cardboard face toward the rotating support S1. Place polarizer P1 between the
laser and the cardboard face. To determine whether the laser beam is polarized,
observe the amplitude of the transmitted beam on screen W1 as the polarizer is
rotated within its holder. If the beam is polarized, the amplitude of the
transmitted beam will change. Is the laser beam polarized? .
Next place polarizer P2 between the screen and the cardboard face, and use
P2 to detect the polarization produced by P1. Minimum transmission results
when the polarizing directions of P1 and P2 are at right angles.
- Snell's law. Remove the polarizers. Place lens L1 (index of refraction = 1.49) on
support S1, centered so that the laser beam is incident on the flat surface of L1 at
the center of curvature. This alignment should be done carefully. The beam
should exit the lens perpendicular to its surface. Rotate S1 and verify Snell's law
[Sec. 6.11, eq. (3)] for a few angles of incidence:
;
(measured) = ;
(Snell's law) = .
;
(measured) = ;
(Snell's law) = .
Insert P1 and vary the polarization. Does
Snell's law depend on the polarization? . - Total internal reflection. Remove the polarizers. Rotate S1 so that the laser beam
is incident on the curved surface of L1. Rotate S1 and observe the angles and
amplitudes of the waves transmitted and reflected from the flat surface of the lens
L1. When the angle of incidence (on the flat surface) reaches the critical angle
for total internal reflection, the transmitted wave emerges parallel to the flat surface of
L1. For
, the transmitted beam disappears, and all incident wave energy is
reflected. Measure the critical angle and compare with the theoretical value from RWVD
Sec. 6.12, eq. (3) or SES p. 272:
(measured) = ;
(theory) = .
Insert P1. Does the phenomenon of total internal reflection depend on the
polarization? .
- Brewster's angle. Insert P1 with the plane of polarization lying in the plane of
incidence (E field parallel to the table.) Rotate S1 so that the laser beam is incident
on the flat surface of L1. Examine the reflected wave as you vary the angle of
incidence
by rotating S1. When
is equal to Brewster's angle
,
then the reflected beam will vanish. Verify this. Measure Brewster's angle and compare
with the theoretical value in RWVD Sec. 6.13 eq. (4), or SES eq. (8.56) (You can remove the
polarizer to measure the angle):
(measured) = ;
(theory) = .
Repeat the above with the E field perpendicular to the plane of incidence (E field
perpendicular to the table). Is there a Brewster angle? .
Next: Lab 7. Antenna Measurements
Up: No Title
Previous: Lab 5. Reflection Coefficient
Michael Lieberman
Sat Aug 15 16:52:53 PDT 1998