Faraday Rotation
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TeachSpin's Faraday Rotation Apparatus, FR1-A, Includes:
- The Light Source
- The Solenoid (magnetic field source)
- The Analyzer Polaroid
- The Optical Detector
The Light Source
The light source is a red laser pointer operating at a nominal wavelength
of approximately 650 nm with a power output of about 3 mw. It requires
a voltage regulated supply of 4 volts and 40 mA. TeachSpin’s audio
amplifier unit AALS1-A has a voltage regulated (4V) supply output specifically
designed for this laser diode light source.
Although the output is approximately 60% polarized, the laser light is
directed through a polarizing filter which increases its polarization
to about 95%.
Before the sample is installed, four nylon thumb screws on the laser mount
are used to aim the laser beam along the central axis of the solenoid.
The entire laser mount is removable so that an experimenter could use
other light sources to study the frequency dependence of Faraday rotation.
If any other sources are used, however, it is important that the intensity
be stable. Small modulations in the frequency will not be as important.
The Solenoid
The solenoid is a 15 cm coil of #18 double insulated wire with DC resistance
of 2.6 ohms. The approximate calibration at its center is:
B = (11.1mT/A) I
where I is in amperes and B is in millitesla.
The magnetic field does vary along the axis of the solenoid. Such variation
may be significant for certain samples, particularly those which extend
outside the coils. We recommend that the students measure the axial magnetic
field profile using TeachSpin’s Hall Effect Probe HE1-A, or other
equivalent field measuring apparatus.
The maximum continuous current through the unit is 3 amperes. For times
of the order of 30 seconds, however, 10 amperes can be used without damaging
the solenoid or its supports.
Since Faraday rotation measurements can easily be performed in 10-15
seconds by a student familiar with the equipment, it is possible to use
these larger solenoid currents and thus obtain larger rotations.
The Analyzer Polaroid
The unit is equipped with a rotatable Polaroid film in a calibrated mount.
The decal is marked in 5° increments. This limits the accuracy of
an angular measurement to about 2°.
The Detector
The detector is simply a photodiode connected in series to one of three
resistors; 10K, 3K, 1K. The photodiode is a current source and is a linear
photonic detector, as long as the voltage across it is less than about
0.3 volt. Saturation begins to occur when this bias voltage appears across
the diode. This makes the detector nonlinear. Varying the load resistor
keeps the bias voltage below the 0.3 volt value
The Sample
FR1-A comes with one sample, a 10 cm long SF-59 glass rod with a diameter
of 5 mm.
The sample is easily removed so that the optics can be aligned.
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