Malus’ Law Purpose

1) Designed for Physics (Grades 11/12)

2) Prior Knowledge: “Polarized or Unpolarized” Activity, Graphing of Trig. Functions

3) Suggested Website:

1) Prior Knowledge: Polarization Activity and Graphing of Cosine Function

2) Vocabulary: Malus’ Law, intensity, electric field and crossed polarizers

3) Suggested Website:

1) Setup  20 min 2) Activity / Lab  20 min

3) Data Analysis  30 min 4) Discussion / Wrap Up  20 min
Materials Needed:

1) VSRT System (See Appendix I)

2) Single CFL set 2 feet from detectors next to each other (~2.5 inches center to center)

3) Polarizers (~2mm wide / ~2mm spacing) made from metallic tape (Cu or Al)

and Polarizer Holder marked at 10° Increments (or mounted to a protractor)
Procedure:

1. 
   Start VSRT System and verify operation without polarizers.
   
2. 
   Place both polarizers in the same orientation and verify signal level above 100K.
   

adjusted near 100K, so the output is roughly the % transmission.

3. 
   Rotate one polarizer relative to the other (in front of separate detectors)
   

4. 
   Repeat until the polarizers are oriented at 90° (crossed) to each other.
   

1. 
   Calculate the % Transmission by dividing the Power at a certain angle by the Power at 0°. { Note: The results should be less than 100% }
   

2. 
   Compute the theoretical transmission by taking the cosine of the angle, then squaring it and multiplying by 100 for a percentage;
   

i.e. – Trans(theo) = cos ²  *100%

3. 
   Calculate the % Difference = (% Transmission – Theoretical Transmission)
   

4. 
   Average the % Difference and place result in the shaded box on lower right.
   

1. 
   Setup the graph for the % Transmission (0% to 100%) vs. Angle (0° to 90°) adjusting the scale of the axes to maximize the size of the graph.
   

2. 
   Graph the measured transmission (2^nd column) vs. angle and label it.
   

3. 
   Graph the theoretical transmission (3^rd column) vs. angle and label it.
   

1. 
   Is the graph of % Transmission vs. Angle linear? Why or Why Not?
   

2. 
   For which angles is there good agreement (<5% difference) between the measured transmission and the theoretical transmissions?
   

3. 
   Where is the % Transmission most sensitive to small changes in angle (few °)?
   

4. 
   Where is the % Transmission least sensitive to small changes in angle (few°)?
   

5. 
   Explain the answers for questions #3 & #4 based on the shape of the curves.
   

1. 
   Vary the polarizer slit widths and spacings to check these effects.
   

2. 
   Try both polarizers in front of either LNBF. Does this also work? Why?
   

Malus’ Law using 12GHz radio waves
	Right Polarizer - Horizontal
		I(max) =	160
	Angle [°]	Signal	Fit=I(max)*cos²
	0	162	160
	10	159	155
	20	126	141
	30	109	120
	40	86	94
	50	60	66
	60	55	40
	70	31	19
	80	18	5
	90	7	0