Stopped at the discount hardware store and found a bunch of measuring tools on sale. This included a digital micrometer with an advertised 1 micrometer precision and a 4 micrometer accuracy for $35. I also picked up some feeler gauges. I need to build a smartphone science microscope next. Then I can image the slits and the feeler gauges or gap in the micrometer in order to measure the spacing and width of the double slit.
The home page of this blog has a picture of fringes from Young's double slit experiment. Although I first did this experiment in high school physics (thanks Mr Roberge), it took me a few days to work out the bugs again some 35 years later. I remembered that to make the double slits, we took two single edge razor blades held together and used them to scribe lines on a glass microscope slide that had been painted black. So, I had razor blades, microscope slides, and an inexpensive diode laser level. The first step was to paint a few microscope slides black. This did not work out so well. The oil-based paint took about two days to dry, and the black was not very opaque. After scribing lines, the paint would close back up again. My next inspiration was to glue aluminum foil to the microscope slide using spray mount glue. This worked very well, and after a few tries, resulted in a high quality double slit. Next was to set up the light source.
Now the original motivation behind this experiment was to resolve the debate as to whether light was a wave or a particle. Sir Isaac Newton was squarely in the particle camp. He passed white light through a prism, and noted that it separated into colors, red, orange, yellow, green, blue, and violet. Then Newton took a second prism, and showed that you could recombine the colors again to get back to white light. Newton rejected the wave theory of light because, unlike sound and water waves, light cast sharp shadows. Light was like tiny bullets (of different color) that traveled in straight lines. Thomas Young (and others) supported the wave theory of light, and developed the famous experiment now known as Young's double slit experiment. If done correctly, light would pass through the two slits and interfere, forming a fringe pattern. OK back to the experiment. Young used a monochromatic source and a single slit first, before the double slit, so that the light at the double slit was coherent. In High school we used a mercury vapor lamp with a green filter (to isolate a single wavelength), and the single and double slits in sequence. You could only do this experiment in a very dark room, as the fringe pattern projected on the screen was very dim. Today, almost everyone has access to a laser pointer, and a cheap laser level costs ~$20. The laser light is already monochromatic and coherent, so the first slit is not necessary. My first attempts to view the fringe pattern was to project the laser into a box with a white screen made from graph paper. The idea was to use my smartphone camera to photograph the fringe pattern. This produced a very poor result. There was lots of extraneous diffraction in the box, and the image came out saturated. I may have been making fringes but I could not tell for sure. Next, I tried to expand the laser beam using a microscope objective. This was also produced a poor result. After mulling things over, I struck upon a technique that worked. I got rid of the box, and instead, projected the laser down a dark hallway to a wall about twenty feet away. The diameter of the laser beam is only about a millimeter, so as long as the double slit is smaller than that, you can simply place the double slit immediately in front of the output of the laser. After a bit of fiddling you can align the double slit so as to produce the beautiful fringe pattern shown in the image. Safety note: looking into a laser is something to be avoided. Keeping the laser at a level different than your eye is a good way to be safe. I ran the laser beam parallel to the floor and a few feet above it. I Since we are all about quantitative measurements here at Smartphone Science, the next step was to measure the fringe pattern spacing, as well as the distance from double slit to the wall. This was easy, Wave theory predicts that the angular fringe pattern spacing (distance between two fringe peaks divided by distance from slit to wall) is equal to the wavelength of the light divided by the distance between the two slits. But what is the slit spacing? and what is the laser pointer wavelength? This I did not know. So how do we measure these? Let's find out! Wavelength can be measured using a spectrometer, and a high quality microscope can be used to measure the slit spacing, but I haven't got these (yet). So future experiments would be to (1) make a spectrometer, and (2) make a microscope measuring station. One other thought came to mind, and that was, is it possible to make a double slit of known dimensions using a laser printer? I will get back to you on that. |
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