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McGee 1
Lab O4
Single and Double Slit Diffraction of Light
Allison McGee
Mr. Fedorchak
Thursday 2:00 4:50pm
Partners: Stevie Whitehead
Alyssa Woods
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Purpose:
To determine the slit width for a single slit and the separation between the two slits
for a double slit.
Introduction
: The wave nature of light was conclusively demonstrated by Thomas Young
in the year 1801 when he was able to make white light diffract by passing a sunbeam
through a pair of tiny slits. This phenomena, a consequence of the Superposition
Principle which states the result of two wave forms superimposed on each other is the
algebraic sum of the two is illustrated below:
In this experiment, we will use a laser to pass through a double slit and single slit
to produce a series of bright and dark spots where constructive and destructive
interference take place.
Double Slit Diffraction – Double slit diffraction occurs when a plane wave of light strikes
a pair of slits separated by a distance “d.” The size of the double slits (openings) has to be
on the order of or smaller than the separation between the two openings in order for us to
observe double slit diffraction phenomena. Graphically:
McGee 3
As the diffracted light waves spreads out, we will begin to see the two “wavelets”
interfere with each other. When the difference in path length (we use the Greek symbol
“
δ
” or “del” to signify path difference) that the two wavelets travel is equal to an integer
number of wavelengths, we get constructive interference. When
δ
is ½ integer number of
wavelengths – we get destructive interference. If we put a screen on the wall a distance
“L” away, we can see the diffraction pattern (series of light and dark spots) caused by the
interference of the wavelets formed by the double slits.
To find the location of the bright spots:
δ
= d sin
bright
=mt; where m=0, +
1, +
2,…
Equation (1)
where
δ
is the path difference, d is the distance between the slits, and
bright
is the
angle from a point normal to the double slits to the bright spot. Note that for small angles
o, we can approximate sin
bright
as
sin
bright
= y
bright
/L
Equation (2)
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where y
bright
is the height from the center bright spot to the next mode (bright spot)
and L
is the distance from the double slit to the screen, m is the mode (an integer), f is the
wavelength of light illuminating the slits. A more convenient form may be to find the
height “y
bright
” above the center bright spot to the higher order bright spots – substituting
Equation (3) into Equation (2), we get
y
bright
=
Lm/d where m=0,+
1, +
2…
Equation (4)
Solving Equation (4) for d, the spacing between the slits, we get Equation (5)
d=
Lm/y
bright
Equation (5)
By setting up a diffraction pattern from a double slit apparatus using a laser of
known wavelength, and measuring the distances to the bright spots and the distance from
the double slit to the screen, we can calculate the distance “d” between the slits.
Single Slit Diffraction – Single slit diffraction is similar to double slit diffraction except
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This note was uploaded on 05/05/2008 for the course PHYS 222 taught by Professor Fedorchak during the Fall '08 term at Campbell University .
 Fall '08
 Fedorchak
 Diffraction, Light

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