7B10.10-Discharge Tubes

7b1010discharge tubes

Video: Watch this demo

Instructor’s video

Description: With this demonstration, each student gets a diffraction grating that they use to view the emission lines of different sources.
The above pictures show what can be expected when viewing each tube though a diffraction grating.


  • High voltage discharge tube power supply
  • Three different discharge tubes
  • Clear incandescent light bulb
  • VariAC
  • Many small diffraction gratings
  • VariAC to banana plug cable

Setup Procedure:

  1. Connect the light bulb to the VariAC. Set to about 120VDC
  2. Be careful when handling the discharge tubes, as they are fragile (as are their cables).

Demonstration Procedure:

  1. Have students come and pick up a diffraction grating.
  2. Start by showing students the incandescent lite. Turn on the VariAC and turn off all other lights.
  3. Have students view the light with the diffraction grating. Note that almost all the frequencies of visible light are present.
  4. Turn off the VariAC and connect one of the discharge tubes to the discharge tube power supply. Go gentle with the cables, they are fragile.
  5. Turn on the power supply and have students view the new source with their gratings.
  6. There are three discharge tubes: Neon, Mercury, and Hydrogen. You can expect students to see the emission lines similar to the above pictures.
  7. Turn off the power supply.
  8. Repeat with the other two discharge tubes. Note the new emission lines characteristic of each gas.
  9. When finished, turn off the power supply.
  10. Have students turn in their diffraction gratings into the plastic bin when done.

Note: The wires are very delicate. The room must be dark so students don’t view other lights in their diffraction grating.