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Fall 07 Phys 143l Practice Exam 1 Solutions

# Fall 07 Phys 143l Practice Exam 1 Solutions - Duke...

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Duke University Department of Physics Physics 143 Fall Term 2007 PRACTICE QUIZ 1 SOLUTIONS Problem 1: Red light from a laser ( λ = 632.8 nm) is incident on a screen containing two very narrow horizontal slits separated by 0.200 mm. A fringe pattern appears on a white piece of paper held 1.00 m away. a. Approximately how far, in mm, above and below the central axis are the ±rst dark stripes? b. How far from the central axis is the 5th bright band? c. Describe/sketch qualitatively what happens to the pattern if more equally-spaced slits are added to the screen. Solution: a. For 2-slit interference, waves are out of phase for d sin θ = ( m +1 / 2) λ , so the ±rst minima are for m = 0. sin θ y/L , where L is the distance to the viewing screen. Solving for y = λL/ 2 d , we get y = 1 . 58 mm. b. For 2-slit interference, waves in phase for d sin θ = . The 5th maximum is for m = 5. y = mλL/d , and plugging in we get y = 1 . 58 cm. c. With more slits, there are maxima at the same places as for the double-slit experiment, and smaller maxima and minima occupy the spaces between them. As the number of slits increases, the main max- ima get brighter and the secondary maxima get both more numerous and dimmer. Eventually, for a very large number of slits, there is es- sentially no light between bright maxima.

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Fall 07 Phys 143l Practice Exam 1 Solutions - Duke...

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