DIFFRACTION OF LIGHT
When light or any other type of wave passes through an opening or goes by the edge of an
obstacle, the wave extends (bends) into the region not directly exposed to the wave front. This
phenomenon is called diffraction. An explanation of this effect was first proposed by Huygens (1629 –
1695), a contemporary of Newton, in the light of a wave theory of light. Huygens’ principle states that
every point on a wave front can be considered as a source of tiny wavelets that spread out in the
forward direction at the speed of the wave itself. The new wave front is the envelope of all the
wavelets. So, when a wave front arrives at an opening, each point along the part of the wave front that
extends across the opening sends out wavelets, all in phase, which spread out in all directions. The
wave theory of light was not always widely accepted. In the early 1800’s, the great scientist Poisson
(1781 – 1840), a wave-theory detractor, pointed out what he thought was an obvious flaw with the
theory, namely that it predicted a bright spot would occur at the center of the shadow created by a
circular object! The experiment was performed and the spot was found!
It is observed that a wave passing through a slit does not illuminate the region beyond the slit
uniformly, but creates bands of high and low intensity. This phenomenon is known as interference,
and occurs when two or more waves arrive at the same point simultaneously. The resulting light
intensity at that point will depend on the relative phase of the component waves. (For example, waves
which arrive in phase add to give a large intensity; waves which arrive out of phase cancel.) In the
Huygens model, the large number of wavelets produced by each of the sources interfere with one
another and result in an interference pattern.
In this experiment, a diffraction pattern is created by allowing the light emitted by a diode laser
to fall onto an aperture. The entire pattern can be observed by placing a piece of paper (or some other
white screen) on the far side of the aperture, or can be examined point by point by having the light pass
through a collimator onto a light sensor. The light sensor is mounted on a carriage which slides
sideways (i.e. perpendicular to the optics bench) using a thumbwheel. There is a two-position switch on
the top of the sensor which controls the gain. (If the signal is too weak, turn it up, and if it is too strong,
turn it down.) The aperture discs consist of the Single Slit Disc and the Multiple Slit Disc and should
be placed in a holder about 3 cm in front of the laser. They carry many slit systems of different sizes
and shapes. Each disc is mounted on a frame in such a way that any of the slits can be rotated in front
of the laser. The frame can be rotated in its mount slightly for better alignment. The entire mount can
be removed from the optical bench for better viewing. A collimator disc in front of the detector is used
to keep out stray light and to better define the position of the incoming beam. The disc has many
collimators of various shapes and sizes to choose from. Choosing a collimator is a balancing act
between intensity and resolution: as the collimator narrows, the spatial resolution increases, but the
The experiment utilizes a computer interface and software (Science Workshop) to collect
and analyze your diffraction patterns. The output of the light sensor is an electrical signal which is
proportional to the intensity of the light falling on the detector. A second sensor in the carriage, called a
rotary motion sensor, monitors the motion of the thumbwheel and so is able to measure the relative
position of the sensor along the rack. The output of the sensors are fed simultaneously to an
interface/software which generate a plot of the intensity as a function of position. 2
Science Workshop Diode Laser and the Collimator Disc
Interface Slit Accessory Holder
Rack with the light sensor
Linear Translator passes through the slot in the side
of the Rotary Motion Sensor
Sensor Slit Accessory
Plan View of the Optics Bench Setup
Figure 1: Equipment Setup with the Computer Interface 3
To prepare the computer and Science Workshop interface, follow these steps:
1. Make sure that the Science Workshop interface is turned on before the computer is turned on.
Otherwise, you will need to restart your computer.
2. Once the computer and interface are turned on, open DataStudio by double-clicking the icon on
3. Click on Create Experiment
4. Click on Analog Channel A and select Light Sensor from the list
5. Set the sample rate to 100 Hz and the sensitivity to 10x 4
6. Now click on the Digital Channel 1 and select Rotary Motion Sensor from the list
7. Set the sample rate to 100 Hz
8. Click the Measurements tab, and enable Position 5
9. Double Click on Graph in the lower left, and select Light Intensity as the data source.
10. Change the graph’s x-axis by clicking on Time and then selecting Position from the list.
Note: a file exists with all of these steps done for you, called “diffraction” on the desktop. You
can double-click this if you make mistakes or get confused. Note that the file is read-only so to
save your data you will need to save to a new file. 6
Exercise 1: Qualitative Study
Never look directly into a laser beam. Avoid accidents by keeping your head out of the plane of the
beam: try to work from above and to the side, looking away from the source. Be alert to the
possibility of reflections from polished surfaces.
Begin by reviewing the description of the apparatus appended to the manual.
What kind of diffraction patterns are you going to see? Turn the diode laser on and place a piece of
white paper over the detector to act as a screen. With the lights dimmed, you can flip through the
various slit systems on the aperture discs while looking at the diffraction patterns on the screen.
Try to discern some of the general features of the pattern. For example, how does the geometry of
the pattern compare to the geometry of the slit? As you go from a narrow slit to a wide slit, what
happens to the pattern? Does the complexity of the pattern correlate with the number of slits?
When you have a good sense of the physical system, move on to the quantitative measurements.
Remove the screen so that the light sensor can monitor the pattern. Instructions for setting up the
computer interface will be available in the lab.
Figure 2: Experimental Setup with a Slit Disk in place 7
Exercise 2: Single Slit
Imagine a coherent light source incident on a single slit of width a and observed on a screen at
some relatively large separation from the slit. The interference pattern from such an arrangement is
shown in Fig.2. Notice that there is a very intense region (called the central maximum) directly
opposite the slit, as might be expected. However on either side of this region the intensity goes to
zero, and away from the central maximum, on either side, are weak