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University of Toronto Mississauga
Wagih Ghobriel

EXPERIMENT DIFFRACTION OF LIGHT Introduction: 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 intensity decreases. 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 Equipment Setup Science Workshop Diode Laser and the Collimator Disc Interface Slit Accessory Holder LIGHT SENSOR LIGHT SENSOR Light Sensor Rack with the light sensor Linear Translator passes through the slot in the side of the Rotary Motion Sensor Light Sensor Diode Laser Rotary Motion 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 the desktop. 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
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