Physical Science

Experiment 8-4

Flame Spectra


Background Information

As mentioned in Experiment 8-3, it has been long known that various substances distinctly color flames.  In common temperature flames the energy excites the metals causing them to glow with distinct colors.  In higher temperature flames, non-metals will also emit distinct colors.

Our eyes normally average together all the colors present in a flame or spark.  This averaging often obscures any overlapping colors that are present.  There are several ways to separate the different colors of light so each can be studied and recorded:prism, grating
  1. The speed for each color of light is slowed a different amount when passing from air into a transparent material such as water, glass or plastic.  If the light does not cross the boundry perpendicularly, the direction of the light changes depending on the shift in speed.  If the light leaves through a parallel surface, in returning to its original speed it will also resume its original path direction.  Howerver if the light emerges a prism (triangular shaped material) at a different angle through the non-parallel surface, the light's path is not restored to its orginal direction so that each color emerges travelling in a different direction.  As a result a viewer sees the light of different colors appearing to come from different directions, making different colors distinct so each can be observed and noticed.

  2. A scratch or narrow slit on a surface causes light to scatter. A series of many closely spaced, parallel scratches on a diffraction grating causes light scattered from each scratch to interfer so that light travelling in some directions cancels away, while it is supportive in other directions.  The result is like a prism in that a viewer sees the light of different colors appearing to come from different directions, making different colors distinct so each can be observed and noticed.

People have probably always noticed the colors of rainbows where light reflected inside nearly round raindrops is nearly all emitted at a surface not parallel to where it entered.  But it was the English professor, Isaac Newton, who was the first to record experiments where he used two prisms, the first to separate sunlight into a rainbow of colors, and the second to reassemble it back to nearly white light.  Newton noted that when he blocked some colors where separated between the prisms, the second prism did not produced not white sunlight, but some color.  Newton concluded that all the colors of the rainbow were needed to make white light.


flame procedureIn this experiment analysis of colors, called spectral analysis, will be used to study light emited by chemical compounds heated by flames.  In principle the colors observed in the spectra could be a combination of that of all elements present, due to just a portion of the elements in the compounds, or perhaps the colors are caused by interactions between elements so are different from that of isolated elements.  This investigation should allow you to determine which of these is correct.

All experiments have some experimental error.  In this experiment some errors are visible and therefore perhaps more apparent that in many other experiments.  The accuracy of the colors you see on the computer monitor probably are not exactly the same colors you would see directly with your eye.  And even your naked eye likely detects colors imperfectly.  The difficulties might be compared to those encountered capturing an experience into language.  In a language, espicially a primative one, there may be no words that accurately convey the complete experience.  So some of the experience is lost in the translation to say ancient Greek.  If the description is than translated to Latin, then to modern English, more accuracy may be lost.  A rainbow of colors has an infinite variety of color like the infinite numbers along a segment of a number line.  But the digital camera only has three detectors, each detecting the intensity of light over a limited range of color.  Like describing the experience using the finite words of the Greek language, much of the description is lost in the capture into three color digital language.  That digital information is accuately moved first from camera to one computer then over the internet to your computer and your monitor.  But the translation by glowing phosphores in your monitor may be less than perfect just like translating from Greek to Latin.  Finallly when you eyes with rods and cones transcribes the color combination into nerve signals for brain interpretation, it is like translating to modern English.  Perhaps this analogy seems unnecessarily complex, but it is important to remember that while every step was as accurate as possible, the colors you see from the photographs below are probably not a perfect reproduction of the flame colors.


Small amounts of various inorganic compounds were held in propane flames by small loops on the ends of metal wires chosen to themselves contribute little flame color.  A plastic grating was taped immediately in front of the lense of a Canon S30 digital camera.  Often the colored flame appeared only briefly.  The following images were selected from nearly a hundred attempts to capture the spectra from a dozen compounds.
Lithium chloride, LiCl:  Some of the light passed directly through the grating allowing a normal photograph of the red flame on the far right.  The grating spreads the colors forming spectra to both the right and left.  But this photograph shows only the spectra to the left.  Note that the light is actually two colors that are merged together in the normal view of the flame.
  1. Record both the color of the flame and the colors of the LiCl spectra.
Strontium nitrate, Sr(NO3)2:
  1. Note the color of the plain flame on the right.
  2. Note the colors of the flame's spectrum on the left.
  3. Note the different intensities of spectrum colors which is given most clearly by apparent flame sizes in the spectrum.  How does the differences in intensities determine the difference when the Sr(NO3)2 flame color is compared with LiCl's flame color?
Cupper II chloride, CuCl2:  This photograph not only includes the plain flame on the far right and the bright first spectral pattern in the middle, but it also shows on the far left a dimmer second spectral pattern wich is often visible.  The supporting wire often glows white hot.  This produce a complete spectrum of all rainbow colors seen roughly under the flame's spectrum.
  1. Note the color of the flame (which often appears much more green; see Experiment 8-3.)
  2. Note the colors of the flame's spectrum on the left which average to green when merged.
Strontium nitrate, Sr(NO3)2: another view from further away.
  1. Note that difference in flame size and brightness compared the the Sr(NO3)2 above.
  2. Note any difference in color.
Calcium chloride, CaCl2
  1. Note the three colors of the spectra; the red is only barely visible here.  (They vertical orange stripe between the flame and spectrum is a reflection off an Aluminum strip attached to the wall in the background.)

Do any of the different elements have identical spectra?

Record your results in your science journal.  Write a Formal Report if you need to earn credit.


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revised 6/3/2003
by D Trapp
Mac made