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
equallyspaced slits are added to the screen.
Solution:
a. For 2slit 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 2slit interference, waves in phase for
d
sin
θ
=
mλ
. 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
doubleslit 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
 Scholberg
 Energy, Photon, Light, Total Energy, blackbody radiation, narrow horizontal slits

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