PhysicsLAB AP Free Response Question
2011 B3 Form B

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A microwave source is placed behind two identical slits, as represented in the diagram below. The slit centers are separated by a distance of 0.20 m, and the slit widths are small compared to the slit separation but not negligible. The microwave wavelength is 2.4 x 10-2 m . The resulting interference pattern is centered on a screen 0.60 m wide, located 2.5 m from the slits.
 
 
 (a) Calculate the frequency of the microwave radiation.

 
(b) On the graph below, where the x-axis represents the distance along the screen and x = 0 represents the center of the pattern, sketch the intensity of the interference pattern expected for that arrangement.
 
 

 
(c) Consider points on the screen located at x = 0.00 m, x = 0.15 m, and x = 0.30 m. Rank the intensity at those points from highest to lowest, with number 1 corresponding to the highest intensity. If two points have equal intensity, give them the same ranking. Justify your ranking.
 
____ x = 0.00 m
____ x = 0.15 m
____ x = 0.30 m

 
(d) Suppose the microwave wavelength is decreased by a factor of three, to 0.80 x 10-2 m. Sketch the resulting interference pattern below.
 
 

 
(e) Suppose the material separating the two slits is removed so that there is now one slit approximately 0.20 m in width. The wavelength is held at 0.80 x 10-2 m . Sketch the resulting diffraction pattern below.
 
 




 
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