Announcements 3/23/12
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xkcd
Prayer
Handouts from last time: Bessel function (for
next lecture), and exam 3 review
Yesterday’s lab: worked until early afternoon…
Reading Quiz
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The array theorem and the book’s treatment of
diffraction gratings are for the __________
approximation.
a. Fresnel
b. Fraunhofer
Reading Quiz
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The array theorem applies to:
a. small circular holes only
b. narrow slits only
c. any identical apertures
Fourier Transform of shifted function
Array Theorem
Reading Quiz
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The resolving power of a spectrometer, l/Dl,
depends on the:
a. number of slits illuminated on the grating
b. separation between slits of the grating
c. both of the above
Diffraction Grating
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“Fraunhofer n-slit” animation
1-D slits, infinitely narrow
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3 slits, equally spaced
(created with parameters so period = 2p)
Back to 1-D slits, infinitely narrow
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4 slits, equally spaced
Back to 1-D slits, infinitely narrow
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5 slits, equally spaced
Back to 1-D slits, infinitely narrow
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10 slits, equally spaced
Back to 1-D slits, infinitely narrow
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20 slits, equally spaced
Back to 1-D slits, infinitely narrow
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100 slits, equally spaced
What have we learned?
The Grating Equation
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Where will maxima be?
Spectrometer
Red light vs. Green light
Resolving Power
N=100
Goes to zero at x=0.0628
= 2p/100
width = 1/N 
separation of
maxima
140
120
100
80
60
40
20
0.4
0.2
0.0
0.2
0.4