63
Diffraction of Light
(Light Sensor, Rotary Motion Sensor)
Equipment List
Qty
1
1
1
1
1
1
1
1
Items
PASCO Interface (for two sensors)
Light Sensor
Rotary Motion Sensor
Slit Accessories
Linear Translator
Aperture Bracket
Diode Laser
Optics Bench, Basic Optics
Part Numbers
CI-6504
CI-6538
OS-8523
OS-8535
OS-8534
OS-8525
OS-8518
Introduction
The purpose of this activity is to investigate diffraction patterns in light and to measure the
wavelength of the diffracted light.
Use the Light Sensor to measure the intensity of the maxima in a double-slit diffraction pattern
and a single-slit diffraction pattern created by monochromatic laser light passing through a
double-slit and a single slit. Use the Rotary Motion Sensor (RMS) to measure the relative
positions of the maxima in the diffraction pattern.
Background
In 1801, Thomas Young obtained convincing evidence of the wave nature of light. Light from a
single source falls on a slide containing two closely spaced slits. If light consists of tiny particles
(or “corpuscles” as described by Isaac Newton), we might expect to see two bright lines on a
screen placed behind the slits. Young observed a series of bright lines. Young was able to
explain this result as a wave interference phenomenon. Because of diffraction, the waves leaving
the two small slits spread out from the edges of the slits. This is equivalent to the interference
pattern of ripples produced when two rocks are thrown into a pond.
In general, the distance between slits is very small compared to the
distance from the slits to the screen where the diffraction pattern is
observed. The rays from the edges of the slits are essentially parallel.
Constructive interference will occur on the screen when the extra
distance that rays from one slit travel is a whole number of
wavelengths in difference from the distance that rays from the other
slit travel. Destructive interference occurs when the distance
difference is a whole number of half-wavelengths.
For two slits, there should be several bright points (or “maxima”) of constructive interference on
either side of a line that is perpendicular to the point directly between the two slits.
The interference pattern created when monochromatic light passes through a single slit is similar
to the pattern created by a double slit, but the central maximum is measurably brighter than the
maxima on either side of the pattern. Compared to the double-slit pattern, most of the light
intensity is in the central maximum and very little is in the rest of the pattern.
PASCO
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63 Diffraction of Light
Physics Experiment Manual
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The equation relates wavelength (lambda), the number of maxima (m), the slit width (d),
m
and the separation of the maxima in the bright fringes of a diffraction pattern ().
sin  
d
The separation of the maxima (), can be determined as follows:
x  L tan 
 x
  tan1  
 L
where x is the measure distance between maxima in a diffraction pattern, and L is the distance
from the diffraction grating to the image of the diffraction pattern.
SAFETY REMINDER: Do not look directly into the laser because doing so can cause eye damage.
Setup
1.
Set up the PASCO Interface and the computer and start DataStudio.
2.
Connect the Light Sensor and the Rotary Motion Sensor to the interface.
3.
Open the DataStudio.

Create a graph displaying light intensity versus the position. The sample rate is set at 50 Hz. The
Rotary Motion Sensor is set for 1440 divisions per rotation.
4.
Mount the Diode Laser on one end of the Optics Bench. Connect the power supply to the
laser.
5.
Place the MULTIPLE SLIT SET into the Slit Accessory holder. Mount the Slit Accessory
holder in front of the Diode Laser on the bench.
6.
Put the rack from the Linear Translator through the
slot in the side of the Rotary Motion Sensor. Put the
rack clamp onto the rack and tighten its thumbscrew.
7.
Place the rack with the sensor onto the Linear
Translator so the back end of the sensor rests on the
upright edge of the base of the Linear Translator. Use
the thumbscrews to attach the rack to the translator.
8.
Remove the ‘O’ ring and thumbscrew from the
Rotary Motion Sensor pulley so they will not
interfere with the Aperture Bracket.
9.
Mount the Light Sensor onto the Aperture Bracket by
screwing the Aperture Bracket post into the threaded
hole on the bottom of the Light Sensor.
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PASCO
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Physics Experiment Manual
63 Diffraction of Light
10.
Put the post into the rod clamp on the end of the
Rotary Motion Sensor. Tighten the rod clamp
thumbscrew to hold the Aperture Bracket and
Light Sensor in place.
11.
Turn on the power switch on the back of the
Diode Laser. Adjust the position of the laser and
the MULTIPLE SLIT SET on the Slit Accessory
so that the laser beam passes through one of the
double-slit pairs on the SLIT SET and forms a
clear, horizontal diffraction pattern on the white screen
of the Aperture Bracket.
12.
Set the slit width “a” to 0.04mm and slit spacing “d” to
0.25mm of the double-slit pattern you use in the Lab
Report section.
13.
Rotate the Aperture Disk on the front of the Aperture
Bracket until the number “2” slit is in front of the
Light Sensor opening. Move the Rotary Motion
Sensor/Light Sensor along the rack until the maximum
at one edge of the diffraction pattern is next to the slit
in front of the Light Sensor.
14.
Once the Rotary Motion Sensor is in place, put the
end stop so it sits up against the sensor. (This is very important because you need to start
each run from the same position.)
Light Sensor
Diode Laser
Rotary Motion
Sensor
Slit Accessory
Procedure
1.
Click ‘Start’. Slowly and smoothly, move the Rotary Motion Sensor/Light Sensor so that
the white screen on the Aperture Disk moves through the diffraction pattern.

Note: It is very important to turn the pulley on the Rotary Motion Sensor very slowly.
2.
Click ‘Stop’ when you have gone though the entire spectrum.
3.
Replace the double slit accessory with the single slit accessory.
4.
Click ‘Start’. Slowly and smoothly, move the Rotary Motion Sensor/Light Sensor so that
the white screen on the Aperture Disk moves through the diffraction pattern.
5.
Click ‘Stop’ when you have gone though the entire spectrum.
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Analysis
1.
Examine the Graph display of Light Intensity versus Position for both the double-slit and
the single-slit patterns. Write a description of the graph in the Lab Report section.
2.
Measure the distance from the double-slit accessory
to the aperture bracket and record this as L in meters.
3.
Measure the distance between the central peak and
the second maxima on the double-slit graph.

Rescale the graph if needed. Use the Zoom Select tool
to select a region from the central maxima over to the
second maxima.

Click the Smart Tool button and move the Smart Tool to
the center of the central maxima. Move your cursor to
the bottom left corner of the Smart Tool. It will turn into a delta symbol.

Drag the delta Smart Tool to the middle of the second maxima.

The delta X is the linear distance between peaks. In the example, delta X is 0.0018553500.
4.
Record the value as x.
Single Slit Diffraction
For single slit diffraction the width of the slit the light passes through is given by the following
equation.
𝑚𝐿𝜆
𝑎=
𝑥
Where m is the order of the dark region, L is the distance between the aperture and the screen the
diffraction pattern forms on, λ is the wavelength of the light, and x is the physical distance
between the dark region and the center of the central maxima. We will experimentally confirm
the size of the aperture by the following setup and procedure.
Setup and Procedure
1. Remove the multiple slit accessory bracket from the bracket holder, and replace it with
the single slit accessory bracket.
2. Carefully remove the rotary motion sensor from the optics track and insert it onto the
optics track the white screen, placing it one meter from the single slit accessory bracket.
3. Using the single slit aperture a = 0.16 mm, readjust the laser so that a clear diffraction
pattern forms on the white screen.
4. Using a metric ruler, measure the distance from the center of the central maxima to the
center of the 3rd dark region on either one of the sides. This length is the physical
displacement of the 3rd dark region from the center of the central maxima (m=3).
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PASCO
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Physics Experiment Manual
63 Diffraction of Light
Circular Aperture Diffraction
Diffraction also occurs when light passes through a circular opening when the size of the
opening is relatively close to the size of the wavelength of the light. The diffraction pattern that
forms on a screen due to a circular aperture is a bright central spot, surrounded by concentric
dark and bright circles. The dark circles correspond to locations of destructive interference and
the bright circles correspond to locations of constructive interference. The diameter of the
aperture the light passes through is given by the equation.
𝐷=
1.22𝜆𝐿
𝑥
Where λ is the wavelength of the light, L is the distance between the aperture and the screen the
diffraction pattern is formed on, and x is the distance from the center of the central bright spot,
and the center radius of the first dark circle. We will experimentally confirm the diameter of
such an aperture using the following setup and procedure.
Setup and Procedure
1. Select the circular aperture on the single slit accessory bracket with the diameter of
D = 0.40 mm, and readjust the laser so that a clear diffraction pattern forms on the
white screen. The screen should still be one meter from the accessory bracket.
2. Using a metric ruler, measure the distance from the center of the central bright spot to
the central radius of the first dark circle.
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Lab Report: Diffraction of Light
Name: ________________________________________________________________
Double Slit Data:
slit width "a":
slit spacing "d":
L = __________ (m)
x = ___________ (m)
Analysis
Calculations
1.
Calculate the angular separation, , for the second peak using the equation:
x  L tan 
 x
  tan1  
 L
2.
Calculate your measured wavelength for the laser using the equation and m =
2 (second maxima):

Remember, d, is the slit spacing value you recorded earlier.
sin  
m
d
Single Slit Data:
Analysis
1. Calculate the experimental value for the size of the aperture. (show work)
2. Using a = 0.16 mm as the accepted value calculate the % error of your results. (show
work)
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Physics Experiment Manual
63 Diffraction of Light
Circular Slit Data:
𝒙 𝟏. 𝟐𝟐𝝀
=
𝑳
𝑫
Analysis
1. Calculate the experimental value for the diameter of the central bright spot. (show
work)
2. Using D = 0.40 mm as the accepted size of the diameter, calculate the % error of your
result. (show work)
Question
How does the plot of light intensity versus position for the double-slit diffraction pattern
compare to the plot of light intensity versus position for the single-slit diffraction pattern?
Extension
The actual wavelength of the laser is 650nm, determine the percent difference between your
measured and actual values.
𝒎𝒆𝒂𝒔𝒖𝒓𝒆𝒅 − 𝒕𝒉𝒆𝒐𝒓𝒆𝒕𝒊𝒄𝒂𝒍
% 𝒆𝒓𝒓𝒐𝒓 = |
| × 𝟏𝟎𝟎%
𝒕𝒉𝒆𝒐𝒓𝒆𝒕𝒊𝒄𝒂𝒍
Please include 3 graphs (single slit, double slit, and circular).
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