Spectra of Gas Discharge Tubes

Here are pictures of the spectra of various gas discharge tubes.  The long skinny discharge tubes, made of glass, are filled with whatever the gas of interest is.  When connected to a several hundred volt power supply, the tubes light up, much in the manner of fluoroescent light bulbs.  The excited gas atoms and/or molecules in the tubes emit light at certain wavelengths (the wavelengths depend on what gas it is).  By noting the wavelengths, and comparing them with some standard - such as these photographs - the type of gas can be identified.  These so-called "spectral lines" are unique to each gas, and depend on the quantuum mechanical details of the internal workings of the atoms and molecules.

I took pictures of these spectra because I couldn't find some of them on the web.   I used a Canon D60 (6 megapixel) DSLR to take these pictures.  I aligned the camera with the viewing aperture of a Project STAR Spectrometer spectrograph (these are great for student or personal use - you can buy them already assembled, or get a kit of 10 of them for students to assemble - go to Learning Technologies for more information).

The spectrograph uses a diffraction grating to spread out the wavelengths of the light that enters the spectrograph through a narrow slit.  Each of the spectral lines (the vertical bars of light with the various colors) is an image of this slit.  A narrow slit gives better spectral resolution (the ability to tell nearby wavelengths apart) but it also reduces the amount of light, making for dimmer lines.

The bottom scale in each frame shows the wavelength of the light in nanometers (a nanometer is a billionth of a meter.  There are a million nanometers in just one millimeter!  The scales were illuminated with a light buld behind the spectrograph.  It's a bit tricky to get the scale bright enough, but not so bright that the spectral lines are washed out.

Note that the colors seem to be a little funky in the region of 590 nanometers - compare the Xenon lines which look red with the Krypton lines which look more yellow.  I suspect these are mostly due to the way the camera's three-color digital sensor interprets colors which are not exactly red, green, or blue.  The line wavelength locations rather than their exact colors should be taken as a better indicator of which gas they represent!

These pictures were used as references for unknown spectra which my students observed during an astronomy lab.  For larger images, just click on the spectrum.  (If you'd like a REALLY big image, let me know!)  You also can save it to a file on your computer.  If you use these spectra, I'd appreciate an acknowledgement.  THANKS!  I can be e-mailed at dbs@astrodave.name .


 

Spectrum of a Helium Gas Discharge Tube

Spectrum of a Hydrogen Gas Discharge Tube

Spectrum of a Nitrogen Gas Discharge Tube

Spectrum of a Neon Gas Discharge Tube

Spectrum of an Argon Gas Discharge Tube

Spectrum of a Hydrogen Gas Discharge Tube

Spectrum of a Xenon Gas Discharge Tube

Spectrum of a Krypton Gas Discharge Tube

Spectrum of a Mercury Gas Discharge Tube

David Shaffer Photography