 |
PHYSICS 220/230
|
|
PHYSICS 220/230
|
Introduction: This week's lab introduces you to interference and diffraction phenomena. We will start by analyzing simulations of water waves in a ripple tank. Then we will make measurements on the optical patterns generated by laser light that is transmitted through single and double slit apertures. We want to be able to explain and predict diffraction and interference phenomena and understand the strong evidence that light is indeed a wave.
The Ripple applet above is designed to demonstrate wave interference effects. Click Edit and move the two sources to positions near the left side or the bottom of the applet. Click on the forward button to observe the simulation. Notice how each point source produces a circular wave emanating from the source. An interference pattern is obtained when the wave fronts emanating from different sources overlap. A distinctive feature of this pattern is the angles at which the waves cancel or reinforce each other. Note these angles, then stop the simulation and edit the ripple tank configuration by dragging the sources further apart. Run the simulation again and note how the angles of constructive and destructive interference have changed. Experiment with a variety of source separations and describe your results. Now use your browser refresh button to restore the original source separation and add additional sources one at a time such that the new sources are aligned and equidistant from adjacent sources. Describe how the pattern evolves.
In order to determine if the water waves in the ripple tank really do obey a double source interference relationship, you must be able to determine the ratio of the source separation to the wavelength of the water waves. With the sources at the bottom or the side, choose a source separation that produces an interference pattern with several well-defined interference maxima and use PrintKey to copy the image to Word where you can print the image. Enlarge the picture in Word before printing, but be careful to maintain the original aspect ratio. You can use a ruler to measure the source separation and wavelength and a protractor to measure the angles corresponding to interference maxima and minima. Show that the pattern is consistent with the general interference rules below by making measurements of maxima and minima in several orders and comparing with expected values.
maxima: minima:
Go to one of the laser stations and find the slide with various double slit configurations. Align the laser beam so that it is incident on the pair of 0.04 mm slits that are separated by 0.250 mm. Project the pattern generated by the transmitted light on a white sheet of paper taped to the wall some distance behind the slits. The pattern should show maxima and minima, suggesting that light is a wave, but the wavelength must be very small in order to produce such closely spaced maxima. Carefully sketch an outline of the pattern on the paper. On another part of the paper, sketch the pattern generated by the pair of 0.04 mm slits separated by 0.500 mm. Measure the distance from the slits to the paper.
When a light beam passes through a single opening, the beam can interfere with itself. This pattern, which is characterized by a broad central maximum, is called a diffraction pattern rather than an interference pattern. You may have noticed the diffraction pattern that was superimposed on the interference patterns obtained in the double slit measurements above. The individual slits produce diffraction patterns that modify the interference phenomenon. You can isolate the contribution due to diffraction by observing the pattern produced by a single 0.04 mm slit. Carefully sketch this pattern on another part of the paper.
It can be shown that the diffraction pattern produced by an obstacle of a particular thickness is equivalent to the pattern produced by an equally thick slit. Sketch the pattern generated by laser light passing around pieces of your and your lab partner's hair.
A) For each double slit interference pattern, measure the average separation of a number of maxima and proceed with the following steps: 1) Combine the maxima separation measurement with the distance between the slits and the paper to obtain the angle between successive maxima. 2) Use this angle together with the known slit separation to determine the wavelength of the laser light. 3) Compare your result to the known value of 632.8 nm.
B) For the single slit diffraction pattern, measure the distance between the first order minima on either side of the central maximum and obtain the angle that is subtended by the central maximum. Use your result and calculate the value of the laser light wavelength using the equation below. Compare your result to the known value of 632.8 nm. (Note the different meanings for theta and d in this equation from those on the previous page.)
diffraction:
Why is θ divided by 2?
C) Analyze the patterns produced by the pieces of hair to determine their thicknesses.
D) If, in parts A) and B), you found any significant differences between the measured laser wavelength and the known one, explain the difference.
 |
Ripple version 4 was written by Wolfgang Christian. |
|