Introduction

Procedure

• Gather these items from your Optics Kit:
  • prism
  • diffraction grating (clear slide thing)
  • red and blue filter
• Gather these items from your home or office:
  • piece of white paper
  • two razor blades
  • a shoebox or other small box
  • tape

• Prism and the spectrum

  a) Take the triangular prism from your Optics Kit and hold it in the sun with white paper as a screen. When you obtain the visible light spectrum (red to blue), note the incident angle and the emergent angle (the emergent beams seems almost at grazing exit to the prism face). Place the red filter and then the blue filter over the illuminated face of the prism to observe the emergent spectrum.

  b) Following Newton's ideas: place the red and blue filters on a white sheet and observe them through the prism with your eyes following the same path as the sun did in a). Do you see colors at the edges of the filter? Look at a continuous line, one half of whose length is red, the other half is blue. When viewed through the prism is the line continuous?

• Make your own spectrometer (Taken from "Seeing the Light")

  A number of common items can be used as diffraction gratings. You can look at the light from a small bright light source reflected by the grooves in an LP record. Depending on the tilt of the record you will see the light source's component colors, provided the source is small or distant enough. How many different orders of interference can you see as you tilt the record? Verify that there are interference beams on either side of the zeroth order.

  Objects with an array of holes that are periodic in two directions can make two-dimensional gratings. Examples are window screens and fine-meshed, thin fabrics. To see the 2-D patterns, however, you need a point source, such as a distant street light or a candle flame. Observe what happens when you tilt your head, when you rotate the grating about the line of sight, and when you rotate the grating into the line of sight. How can you tell which of two gratings have the finer mesh, whether the grating constant in the two directions is equal, and whether the two directions of periodicity are perpendicular? (Try distorting a piece of screening and see what happens to the pattern.)

  You can use the diffraction grating in your Optics Kit or purchase replica gratings cheaply -- as "spectrum glasses" in novelty stores (coarse gratings) or from scientific supply houses. For a bit more you can purchase gratings made to be used as camera "filters" for special effects, such as rainbow (line grating) and crostar (square grating) filters. The scientific filters specify the grating constant by telling you that the grating has, say, 13,400 lines/inch. (So in this case d = 1/13,400 in = 2.54/13,400 cm = 0.0002 cm = 2,000 nm.) Other types should be calibrated using a known wavelength, such as the strong blue line of mercury, which has = 426 nm. If you do not have a mercury lamp (sun lamp) you can see this line in a black light or as a brighter line againsta continuous background in the light from a fluorescent tube. You can get precision spectra if you mount your grating so that it is illuminated by a narrow slit and stray light is excluded.

  

  Interesting spectra to observe are those of a light bulb, of fluorescent lights, of black lights, of sodium lights used for highway illumination, and of "neon" lights used in bars. Also note the effect of colored filters or of a glass of colored liquid held in the light beam.

  You can photograph the grating pattern by holding the grating closely in front of your camera lens, focusing and exposing as you would normally. Scenes with sharp highlights acquire new colors. For colored rings around each bright spot, such as Christmas tree lights, rotate the grating about the line of sight while taking a time exposure. It does not matter if you jiggle the grating a little while rotating it, but the camera should be firmly mounted on a tripod. Why do you get rings, and why are they centered around the bright spots (rather than the center of the picture)?

  Build the spectrometer shown in the diagram above. The slit made by the razor blades should be very thin (less than a few millimeters across) and your box should be relatively light-tight. Be sure to mount the diffraction grating so that the spectral lines are seen vertically.

• Calibrate your spectrometer.

  a) Use a ruler to draw a wavelength scale along the bottom right-hand side of the back of your spectrometer box (see diagram above). Make sure the spacings are equal.

  b) Look at a fluorescent bulb (in the ceilings of most office/academic buildings) with your spectrometer. A line spectrum should appear on the right-hand side as you look through the grating: a bright green line superimposed on a fainter continuous spectrum. This bright green line is a mercury line at 546 nm. Mark this point on your wavelength scale, and label the rest of your scale, with the longer wavelengths (lower energy -- red) on the right side and shorter wavelengths (higher energy -- blue) on the left side. The bluest line you will see (indigo) is at a wavelength of about 400 nm, and the reddest line you will see (red) is at a wavelength of about 700 nm.