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UW-Madison PHYSICS 208 - Laboratory

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LC-1: Interference Name_____________________________ Lab #1 Worksheet Section___________ 1 Your TA will use this sheet to score your lab. You need to turn it in at the end of lab. You should use complete sentences and clearly explain your reasoning to receive full credit. The light source is a laser diode, with a wavelength of about 670 nm. Carefully read the laser handling precautions in the lab manual: the laser diode is a source of extremely intense light, which can damage your or someone else’s vision if mishandled. Your experiment consists of Part A: Interference and Diffraction 1) A laser diode light source, 2) A circular wheel of single slits of various widths, 3) A circular wheel of double slits of various widths and spacings, as well as 4) A diffraction grating mounted to a holder that fits on the optical track 5) A light detector. The light detector also has a circular wheel in front of it, with apertures of various larger widths. You move the light detector perpendicular to the beam path to quantitatively measure intensity variations of the interfering or diffracted light. The different slits are to set the spatial resolution of the light detector: slit #1 works well, with a sensitivity setting of “10” (slide switch on detector). If this is too sensitive (so that the detector saturates, as evidenced by flat tops to the diffraction peaks), you can use “1”. 6) An optical track on which all these pieces can be mounted. Part B: Using interference to measure small spacings 7) A CD and a DVD Part C: Thin film interference 8) A Luminesque Fireworks Bow, and other multilayer plastic cut from a lower-quality bow. 9) A piece of dichroic “art glass”. This is a glass substrate with thin-film multilayers on top. Part A. Interference: Start out with Experiment II in the lab manual Here you will be investigating two-slit interference. The two slits illuminated by the laser beam act as two sources of spherical waves of light that are of the same frequency and same phase. These spherical waves form an interference pattern throughout all space. That is, at all points in space, the total light is a superposition of light originating from the two slits. You investigate the interference pattern visually by looking at the reflection from a white screen, and you record quantitative information on the computer with a light detector that you move manually across the interference pattern. Setup: Position the “Multiple slit set” circular wheel at 110 cm on the track, and the laser diode directly behind it (this barely fits on the track). Make sure that the actual disc lines up with the 110 cm mark, and not just some part of the plastic holder. This sets the slit-screen distance to 100 cm. Turn the multiple slit wheel to position the “a=0.04 mm, d=0.25 mm” double slit in front of the laser beam. Position the detector aperture (labeled “1” or “2” on the aperture wheel) near the center of the track. If the diffraction pattern doesn’t fall on the detector aperture, use the thumbscrews on the back of the laser to adjust its angle Put the white screen on the track in front of the light detector.LC-1: Interference Name_____________________________ Lab #1 Worksheet Section___________ 2 A1) Describe or sketch what you see on the white screen. A2) The wavelength of the diode laser is about 670 nm. Calculate approximately how many wavelengths of laser light are in the space between the slits and the white screen. A3) Now take off the white screen, and start the DataStudio program on the lab computer by clicking once on the settings file on the “Laboratories” page of the course web site (not the lab manual web site). Use the computer to record the intensity pattern: click start and slowly move the photodetector across the interference pattern by turning the wheel with your hand. Enlarge the data in DataStudio so only the central peak and two peaks on either side fill the screen. Record the distances between the central maximum and the immediately adjacent minimum, and the next maximum. From central peak to nearest minimum From central peak to neighbor peakLC-1: Interference Name_____________________________ Lab #1 Worksheet Section___________ 3 A4) Here is a sketch of your experimental setup, but not to scale. The slit spacing d is much smaller than shown, and the screen distance L is much larger than shown. The two slits act as two in-phase sources of light. The waves travel out in all directions, and hit the screen in all places – the lines are just to indicate the path of the light that hits the detector at point x. We showed in class that one path is longer than the other by an amount ! "= d sin#, where θ is the angle of the detector away from the dashed centerline. Using this relation, find the path length difference for the maximum and minimum in the previous question. (Hint: ! tan"= x / L) 1st minimum Next maximum θ δ in millimeters δ in nanometers δ in wavelengths of laser light A5) The waves from each slit start out in phase, but they propagate difference distances to reach the detector. What is the condition on δ that they Interfere constructively? Interfere destructively? d L Detector Slits x ! "= d sin#= path length differenceLC-1: Interference Name_____________________________ Lab #1 Worksheet Section___________ 4 How does this compare with the results in your table above? A7) Find the spot on the wheel that has groups of 2, 3, 4, and 5 slits. These all have the same slit separation, and slit width, just different numbers of slits. Take data on the computer for each of these. This will result in four graphs. Examine the graphs by zooming in to the central 5 peaks of each. If you always start the detector at the same spot, and move it in the same direction, the graphs will be mostly aligned with each other on the computer screen. Do the following aspects of the interference pattern increase, decrease, or stay the same as the number of slits increases? Separation between max Width of max Height of max Increase, decrease, or stay the same A8) In this part you use a ‘replica grating’. This is an extended version of the multiple slits above, with ‘slits’ ruled continuously across it at a density of 600 lines/mm. Shining the laser beam through


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UW-Madison PHYSICS 208 - Laboratory

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