Shannon's

                                                            Diffraction Grating

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    Our physics class did an experiment which involved diffraction gratings. Diffraction gratings look like square pieces of plastic. However, they actually have thousands of grooves lined up on their surface. Each of these grooves acts like a slit in the single and double slit diffraction grating experiment we did. To begin studying diffraction gratings, we first looked through a grating with our eyes. This produced a streak of colors in rainbow order. This is because the grating separated the light into its component parts. The grooves had to be running up and down, however, to make this work.
    Next, we used the grating to observe the light produced by the lamps of several different elements. We found that the light from a Hydrogen lamp produced three distinct bands of color when viewed through the grating.  The light was separated into a purple line, an aqua line, and a red line. Helium light produced four distinct lines. A blue-violet line, a green line, a yellow line, and a red line were clearly visible. There were also vague lines between the colored lines. The Neon lamp glowed bright orange. There were many orange and red line produced, and there was also a vague green line. Mercury light porduced a purple line, a green line, and a yellow-orange line. Visit the Spectra of Gas Discharges page for some good pictures of elemental spectra.
  After we had observed how diffraction gratings worked, we perfomed an experiment to measure the wavelength of laser light using a grating of a known pitch, or number of slits per millimeter.  The grating we used had a known pitch of 600 slits per millimeter. The set-up looked something like this: 

    The light shining through the grating was diffracted at a certain angle. Using the knowledge that pitch= 1/PD, we used the formula below to find the wavelength of the laser light.

                                                            l = sin q d
    In the next part of the experiment, we took the known wavelength and found the pitch of an unknown grating. The same equation and setup were used. A table of the results is shown below.
 

    Each pair of adjacent grooves can be viewed as a double-slit diffraction, and so the same equation can be used to calculate the diffraction angle. When the laser was shone through the grating, a strong center dot was formed, and two other dots could be seen on either side of the center dot.  The second dots are first order diffraction. There is a pair of dots a little further out called second order diffraction. The pair of dots even further out are third order diffraction. These diffraction dots are caused by the infinite wave fronts produced by each wave as it goes through a slit. The diagram below (from the online version of our lab handout, at http://www.warren-wilson.edu/~physics/physics2/Grating_Spectra/Grating_Spectra.html )
 
 

    Finally, we used the set-up below to determine the wavelength of the colored lines in the Hydrogen spectra. We measured the distance from X cen for each color, divided that number by D to get the tangent of the andgle, and then used the known d and the equation above to find the wavelength.