FARADAY EFECT AND POLARIZATION - Auburn University



FARADAY EFECT AND POLARIZATION (Rev 8/2/11)

Purpose: Verify Malus’s Law and study Faraday Effect. Determine Verdet Constant.

Background: Use Internet or optics text to read about Malus’s Law for polarized light. Read about Faraday Effect in Melissinos book (on shelf in lab), on Internet or in an E&M text.

Equipment Safety: Do not connect or disconnect laser from power supply while power turned on. If using power supply for laser other than the one specified below do not exceed 4v to laser. Take precautions to avoid applying large electrostatic voltage to laser power leads such as keeping wrap bare wire wrapped around leads of laser when not in use.

Part I Malus’s Law

Setup

1. If the glass sample is still in the solenoid you will have to disconnect the Polaroid filter, laser and detector to remove glass sample. Use a Kleenex on end of a pencil eraser to remove it. Push it out slowly. Put it in the small white box. Replace the Polaroid filter, laser and detector after sample removed.

2. Connect a digital voltmeter to detector output. Put detector switch in 3K position. The voltage will be < 0.3v when using detector.

3. Connect the output connections on the back of the Teach Spin Amplifier/Power supply to the laser. Plug in. Rotate Polaroid filter for maximum voltage reading. Rotate laser slightly for maximum reading. If voltage < 0.2v have instructor check alignment.

Taking Data

4. Look for small white line on Polaroid filter. Adjust filter for maximum voltage (but below 0.3v). Read angle opposite white line. Record angle and meter reading.

5. Rotate filter in 5 degree increments up to 90 degree rotation. Record angles on scale and meter readings. Repeat rotating the other way. The voltage will be a measure of the intensity of the light.

6. Return filter to maximum voltage position and repeat data taking twice more.

7. When finished turn laser voltage to zero. Turn off power supply and meter.

Part II Faraday Effect

Setup

1. Connect the PASCO power supply to solenoid. If available use radial probe and the gaussmeter to determine the field in center of solenoid at 1 amp intervals from 1-7A. Turn current back to zero between measurements for 4-7 amps. For 4-7 amps get measurements in less than 20 sec each. If the gaussmeter with radial probe is not available use the formula after analysis section to estimate value of B.

2. Remove the filter, laser, and detector and carefully install sample using Kleenex on end of pencil eraser to insert sample. The white “spacers” are left on the sample to center it in solenoid

3. Reinstall filter, laser and detector and repeat steps 2 and 3 in Part I

4. The magnetic field will rotate the angle of polarization by and amount that depends on the filed strength and the material. Raise solenoid current to 3A while observing the voltage. It should change. Reduce solenoid current to zero.

Data Taking

Method I

5. Rotate polarizer until you get maximum voltage. Record angle. Estimate to 0.5 deg.

6. Then rotate 45 degrees either way. Record voltage. Increase current to 3A and

determine angle needed to restore 45 deg voltage reading. Let solenoid cool off before

doing next steps.

7. Repeat steps 5-7 above but using 5 A. Get each angle within 15 sec. Rotate polarizer so voltage is close to 45 deg value. Then turn current back to zero and let solenoid cool off while reading angle.

8. Repeat steps 5-7 using 7A. Rotate polarizer so voltage is close to 45 deg value. Then turn current back to zero and let solenoid cool off while reading angle.

9. Turn solenoid current to zero when finished with measurements.

10. Repeat steps 5-9 at least two more times so you have at least three measurements at each current.

Method IIA (Red Laser)

1. Turn on equipment. Reduce solenoid current to zero.

2. Adjust polarizer for maximum voltage.

3. Rotate polarizer to the right by 45 deg.

4. Record voltage using all digits on DVM (= I45)

5. Record voltage for 2 thru 7 amps plus highest current you can get. Do not waste time trying to get exact currents. Get close and record values. Turn current back to zero when finished taking data.

Method IIB (Green Laser)

12. Unplug laser power supply and disconnect from laser. Unscrew the laser from the.

board Use rods and clamps to hold the PASCO laser. Align the green laser beam with

solenoid and detector. Adjust polarizer for maximum voltage. You may need to use a

different switch position on the detector to get a voltage between .2 and .3v. Get data

for the green laser using procedure of method IIA. Put red laser back in place when

finished.

13. When finished with experiment turn off voltmeter, and solenoid power supply Unplug

Laser power supply and disconnect. Disconnect meter and solenoid power supply.

Carefully remove sample and put in small white box.

Part I Analysis

1. For each angle (0, 5, 10 etc) compute cos2(angle). Compute I/Io where Io is voltage at 0 degrees.

2. Plot cos2 versus angle and draw a smooth curve through the points. Plot your intensity ratio values versus angle on the same graph. Use different colors or symbols for each data set.

3. How well do your results fit theoretical curve?

Part II Analysis

Method I

1. Use the formula below to compute the Verdet constant (Cv) of the

sample for the 3 currents. The angle in the formula is the angle through which you had

to rotate polarizer to restore the initial voltage in steps 5-8 above. Compute this angle

in radians. The length of the sample is 10 cm. Use field in T and length in meters in

formula.

Angle = Cv x B x L

2. Compute average value of Verdet constant and compare your Verdet constant value

with “book” value. You may have a large difference due to the small size of the effect

and limited ability to read angle. The Verdet constant for the SF-59 glass sample is

given on Teach Spin web site in the description of the Faraday Effect experiment.

3. Why were the measurements taken at 45 degrees rather than at maximum or minimum?

voltage? Think about Malus’s law.

Method IIA

4. Use I = 2 I45 cos2(A) to get angles where I is the voltage value. Then get A – 45.

5. Using gaussmeter data or formula below plot angle/length versus field in T where rotation angle = A – 45.Find the slope. This will be the Verdet constant.

6. Compare your Verdet constant value with value given on Teach Spin web site

Method IIB

6. Calculate Verdet constant as described above and compare with value for red laser. Is

there a significant difference or about same value? The wavelength of the red laser is

650 nm and the green laser is 532 nm. Use “SF-59 glass” to search for a web site that

discusses variation of Verdet constant with wavelength.

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Use this if gaussmeter with radial probe not available. If a probe is available use this formula to check your results.

B = (11.1mT/A) x I

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