Dark spots in the Rainbow – Red shift blue shift and seeing absorption lines

Astronomy makes many claims about Doppler shift, gravitational red shift and assertions about the composition of celestial objects yet all we ever see from them is light. All of these assertions depend on the evidence of spectral lines in the light we receive. Many of us may have seen spectral emission lines in the classroom or with toy rainbow glasses but there is an additional jump from this to the absorption lines seen within the spectrum of a star like our own sun. In this video I tackle what it takes to see absorption lines for yourself.

Paper tube (large and small), black velvet or felt fabric, CD, Duct Tape, razor blade or Xacto knife Mechanical pencil refills (graphite sticks), glue gun, chrome plated screwdriver or rod, digital camera with exposure adjustment, tripod (optional), rubber bands (optional).

Experimental steps:
There two experiments with similar procedure but one is much smaller and less accurate and cannot see absorption lines while the other is larger and should be able to show absorption liens.

Simple analyzer:

  1. Start by making a clean slit in a card, this can be achieved several ways, by aligning razor blades, cutting a piece of aluminum foil, but I use the method discussed in my Quantum Basics I video.
  2. Cut a simple hole in the card and tape two pieces of graphite pencil refills approximately half a millimeter apart across the hole.
  3. Carefully place more tape so that the only remaining opening is that of the slit between the two pieces of graphite. This will be your slit for either spectrum analyzer.
  4. Next tape this onto one end of a paper towel tube. Be sure to use the tape to block out any light other than from the slit.
  5. Now you need to make a diffraction grating. Take a CD (new or used it doesn’t matter) and score a few cuts on the less shiny side of the CD (this can also be done with a DVD), where the label would go, with a razor blade.
  6. Then over the area where you scored put some duct tape and press down thoroughly.
  7. Peel the tape up and some of the silver  covering should come off (if it doesn’t score again or try a different location on the CD).
  8. Repeat until you have a window similar in size to the opening of the paper towel tube.
  9. Now align the CD with the slit you made earlier so that the tangent of the curve of the CD is parallel with the direction of your slit (if that doesn’t make sense just watch the video).
  10. Once aligned, cut to size and tape in place. It is important to make this alignment ad well as you can for best results.
  11. Now simply hold the tube up to your eye or digital camera with the CD near your eye/lense and point at various sources of light.
  12. When looking at a light source off to the side you should a see brightly colored rainbow spectrum. If you cannot or the spectrum looks blury, check your alignment or make your slit more narrow.


  1. For the large tube capable of seeing an absorption lines you will make the same slit and diffraction grating as described above (steps 1-4 for slit and steps 5-10 for diffraction grating)
  2. However, in the tube there several tricks to help make the lines more detailed. First you are using a longer tube, this reduces scattered light coming down the tube from reaching the eyepiece.
  3. Next the inside of the tube should be made black. The best way to do this is line it with cloth since the fibers of the cloth are less likely to reflect light around the tube. Cut a piee of black velvet or felt to the length and circumference of the tube, giving yourself a bit of extra all around.
  4. I found it works well to wrap the cloth you intend to use on the outside of the tube and glue the cloth to itself with a glue gun.
  5. Once you have a cloth sleeve, slide it off the tube and insert the cloth sleeve into the middle of the tube and fold over the ends. Glue one end in place and leave the other end open (you see why in the video).
  6. Now take caps of some sort (mine came with the poster tube I was using) and make sure they have appropriate holes in them and are colored black.
  7. Place caps onto both sides of the tube and preform the same alignment of the slit and CD diffraction grating as for the smaller version. Be sure you put the CD diffracting grating on the side with the loose cloth.
  8. Next put your screwdriver or other chrome plated rod at an angle to reflect the suns light (don’t want to damage your eyes or camera).
  9. Point the tube at the reflecting rod, and ideally hold in place with a tripod or other fixed support with tape or rubber bands. This leaves your hands free to manipulate a camera.
  10. Now place the camera up against the CD and cover the sides as best you can with your hands, or the loose cloth to block out excess light.
  11. Again once everything is aligned you should see off to the side a colorful spectrum.
  12. Adjust your camera for dim light with a long exposure, this will help make the absorption lines more clear.
  13. This may take a lot of fiddling and alignment but it should work eventually.

Data: Here are the images I took of various light sources with the simple spectrum analyzer.

flourscent 2

Here is my initial test from the CFL lamp on my desk. You can easily see different lines from the phosphorescent powder on the inner lining of the tube.

Below are several other light sources I looked at.

Street Light
Street Light
birthday candle
Birthday Candle
tea candle
Tea Candle


screen 2
LCD Screen


After playing around with this for a while I proceeded to take pictures with the same camera but now using the more accurate larger tube. Here are the raw unedited .jpg image files.

IMG_5463 IMG_5462

IMG_5462 IMG_5467 IMG_5454



At the thumbnail size they may be difficult to make out but several of these do show the absorption pattern of some of the strongest lines. The first one in particular I was able to make out lines that correspond with sodium, magnesium and calcium. There do also appear to be a few hydrogen related dips but those were not as clear. Still I find this convincing. Let me know if you try this of if you see some lines I didn’t in the images!



Yellow Street Light – Sixty Symbols

About spectra in astronomy

Diffraction angle for CD, vs DVD, vs blue ray

Youtube videos on construction

Different setup with a long tube and a cylindrical screwdriver as a sharp line source. Camera placed at top of tube looking down.

For spectral lines on slinkies

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