Assignment 10
Due: 11:59pm on Thursday, July 7, 2011
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± Understanding TwoSource Interference
Learning Goal:
To understand the assumptions made by the standard twosource interference equations
and to be able to use them in a standard problem.
For solving twosource interference problems, there exists a standard set of equations that give the
conditions for constructive and destructive interference. These equations are usually derived in the context
of Young's double slit experiment, though they may actually be applied to a large number of other
situations. The underlying assumptions upon which these equations are based are that two sources of
coherent, nearly monochromatic light are available, and that their interference pattern is observed at a
distance very large in comparison to the separation of the sources. Monochromatic means that the
wavelengths of the waves, which determine color for visible light, are nearly identical. Coherent means that
the waves are in phase when they leave the two sources.
In Young's experiment, these two sources corresponded to the two slits (hence such phenomena are often
called twoslit interference). Under these assumptions, the conditions for constructive and destructive
interference are as follows:
for constructive interference
,
and for destructive interference
,
where
is the separation between the two sources,
is the wavelength of the light,
is an arbitrary
integer, and
is the angle between a line perpendicular to the line segment connecting the sources and
the line from the midpoint of that segment to the point where the interference is being observed. These
equations are often spoken of in terms of visible
light, but they are, in fact, valid for any sort of
waves, as long as the two sources fit the other
criteria given.
Part A
Which of the following scenerios fits all of the criteria for the twosource interference equations to be
valid?
ANSWER:
An observer is standing far away from two red signal lights.
Light from an incandescent bulb shines onto a screen with a single slit;
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View Full Documentthen the light shines onto a screen with two slits in it and the light from the
two slits finally shines onto a faraway screen.
An observer stands on a road far away from two neighboring radio towers
for different radio stations.
Light from an incandescent bulb shines onto a screen with a single slit;
then the light shines onto a screen with two slits in it and the light from the
two slits finally shines onto a nearby screen.
An observer stands on a road that runs five kilometers away from the two
synchronized transmitting towers for a radio station.
Correct
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 Summer '11
 Bratton
 Light, Wavelength, Malus

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