Physical Science

Experiment 8-5

Spectra of Sparks


Background Information

In the 1850s physicist Gustav Kirchhoff (in Königsberg, Prussia) and chemist Robert Bunsen (in Gottingen, Germany) invented a spectroscope which separated the colors of light.  They used spectroscopes to systematically study the effects of earthly substances on the color of flames.  In 1859, they found each chemical element emitted unique spectral lines which were not effected by the presence of other elements.  Therefore such spectral lines could be useful for identifying even trace ingredients of unknown materials.  Using this technique, Robert Bunsen found two new elements, one he named Cesium for its blue spectral line, and the second named Rubidium for its red spectral line.

Another method to create spectra is to bombard a gas with a high voltage electric spark.  It was noted a kind of negative spectra could also be obtained by passing white light through a container of room temperature gas.  Kirchhoff described the relationship between three types of spectra as:

Observing and comparing spectra is valuable for identifying elements under a variety of circumstances.  Unlike traditional chemical analysis, spectral analysis requires only miniscule quantities of substance.  Alternatively, if the spectra is bright enough, the observer can be astronomical distances away.  As a result of these features, spectral analysis is used for criminal investigations, discovery of new elements, and studies of the most distant parts of the universe.


spectra apparatus

In this experiment such high voltage electric sparks are passed through several gases.  The spectra created by a diffraction grating are photographed so you can observe and compare them.

The apparatus (in the middle of the photo at right) contains Neon gas in a vertical glass tube (about 25 cm long), glowing due to the passage of a high voltage spark generated using a transformer in the blue housing.  A diffraction grating has been placed directly in front of the camera lense.  The grating used at right consists of piece of clear plastic containing a hologram of an evenly spaced vertical grid with a spacing of 13,500 per inch (about 530 per mm).  Interference caused by the grating diverts some of the light to the left and some to the right so that vertical stripes of color appear to come from either side of the apparatus.  The interference deflects different colors of light different angles causing slightly different colors to appear next to each other.  In the case of Neon, there are several shades of red, orange, and yellow slightly separated by dark gaps.  Distinct colors have different intensities: in this case the deep red is fainter.  The diffraction grating fails to deflect some of the light which comes straight from the glass tube in the center of the apparatus.

Camera mechanisms are different from the mechanism used by human eyes to detect colors.  See Primary Colors: Not and Color Vision: a Dialogue in the Style of Galileo.  The colors seen by eyes and cameras are approximately the same.  However the eye typically adjusts to bright light but tires over time resulting in a changing perception of color.  Camera film and charge coupled devices used in digital cameras don't tire, but instead respond differently to different intensities: bright light often appears whiter.  For example, the light coming directly from the glass tube in the apparatus (middle of photograph) looked an orange-red to the eye, not white.

Second, third, and fourth diffraction patterns which duplicate the pattern of colors but are dimmer occur further away from the apparatus.  Details of the portion of Neon's spectra visible left of the apparatus are shown below.  A similar but reversed mirror image occurs to the right of the apparatus but wasn't photographed.  The holographic grid in the diffraction grating used for this detail and all the details below has a spacing of 1000 per mm, providing greater separation of colors.

Ne spectraBecause of deficiencies of the mechanism used by the camera to record light, the three duplicate Neon patterns don't appear identical.  The brightest light (on the right) blurs the details and whitens the first pattern.  Note that what should be an identical second pattern is more distinct than the first, but the third pattern (far left) is nearly too faint.  The faintest lines (Neon's green) are often only visible in the brightest first pattern.  By comparing the duplicate patterns, it is often possible to detect spectral lines lost by light too bright or dim for the camera.

various spectra To the right are a series of spectra patterns for six gases.  These were obtained by the same digital camera (Canon S30) and apparatus shown and described above.  The photographs were taken in a darker room, then cropped in PhotoShop and aligned so identical colors are approximately aligned vertically.  Two patterns were photographed for Helium (He), Nitrogen (N), Neon (Ne), and Mercury (Hg) with the second pattern starting roughly in the middle, excluding label. Perhaps it should be noted here that the 50 year delay in explaining the complicated spectra patterns was not due to inattention by scientists.  Many scientists studied spectra and were frustrated.  As is often the case with complex phenomena, understanding one phenomena may require the development of seemingly unrelated concepts.  In the case of understanding the cause of atomic spectra, the following were required prerequisites:
  1. Establishment of the reality of atoms (many contributors)
  2. Development of energy as a conserved property (many contributors)
  3. Understanding of electromagnetic radiation (J. Maxwell)
  4. Discovery of the electron (J.J. Thomson)
  5. Invention of the quanta explaining black body radiation (M. Planck)
  6. Use of quanta to explain the photoelectric effect (A. Einstein)
  7. Discover of the nucleus (E. Rutherford)

Perhaps this is also a good occasion to note that while science does not prove ideas true, when all the pieces are shown to be logically consistent and able to explain detailed evidence such as the spectra above, scientists gain a great deal of confidence that their ideas must be correct.  The acknowledgement by scientists that they never know for sure should NOT be regarded as inferior understanding to that claimed by those who have absolute answers justified by dogma.

Record your results in your science journal.  Each spectra might be better preserved with a diagram rather than exclusively described in words.  Write a Formal Report if you need to earn credit.


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revised 6/1/2003
by D Trapp
Mac made