Kirchhoff and Bunsen were first dazzled by the interaction of matter and light 152 years ago. While a long list of scientists and discoveries contributed to their level of investigation, Kirchhoff and Bunsen were likely the first true spectroscopists.
One of the principal things that interest me is the interplay between light and matter. It’s the basis for radio communication, harnessing energy from our sun, and an incredibly accurate quantitative measurement. As a child growing up, I had an insatiable hunger for knowledge on the subject, which is not quite available to a child. My father would offer up his explanations and I would try to extract information from the Internet, but nothing was digestible to me.
This illustrates an important motivation for designing this demo (below). In my search for answers on the nature of light and its interactions with matter, I was exposed to ideas on relativity, quantum mechanics, chemistry, and physics. These were necessary to understanding light, at least understanding it as much as possible. Later, when I had to study these topics in school I was excited to put in the work.
By the time I reached college and started studying chemistry, I was surprised how quickly the fine details of spectroscopy were skipped over. Students are taught to take for granted the subtle interplay between photons and atoms. I hope the demonstrations listed below will help your chapter demonstrate these fantastic principles. This is not meant to be an end-all guide, but an inspirational and fun note on self-discovery.
There are two simple experimental set-ups useful in illustrating the basic principles of spectroscopy. The first simply involves passing light through homemade samples and noting which phenomenon is occurring: absorption, scattering, or emission. The second experiment focuses on how chemists design an instrument in general, and then uses that equipment to support inquiries about the physical world.
Light and Matter Demo
- A set of red, green, and UV/blue laser pointers
- Glass sample containers
- Solvent (water or ethanol)
- Samples (highlighter, food coloring, and milk)
- Begin by filling your sample containers with solvent. Most household samples will be water-soluble.
- Dissolve a different sample into each sample container. This can be done by rubbing the highlighters tip vigorously against the bottom of the container until the solution changes color. You can also make sample solutions by adding food coloring to your solvent. A very dilute solution of whole milk will also serve as a light scattering example. As always be sure to save one container of solvent as a blank (no sample).
- Try to develop a hypothesis before analyzing your samples. You should be able to predict which sample will absorb your chosen laser.
- Test out your various laser pointers by passing them first through the blank and then into your sample container.
What is going on here?
The first thing is to note the use of a laser. Lasers emit a coherent beam of monochromatic light (a narrow distribution of wavelengths). When you pass your red laser through a solution of green dye we see total absorbance; in general, green materials absorb red wavelengths of light and reflect or scatter green wavelengths back towards our eyes. Secondly, these demos can be expanded to illustrate emission from a sample. By using your blue/UV laser in conjunction with a solution of highlighter dye, you will see a brilliant luminescence from the solution. The fluorophores within highlighter dye absorb and emit much more intensely when stimulated directly with UV light. To take this demonstration further, coat a piece of cardboard with glow in the dark paint. Several seconds after excitation with your blue/UV laser you will continue to see light emitted from the paint. This is a special case of emission called phosphorescence. The concepts behind phosphorescence are closely related to fluorescence but certain processes take a longer time, leading to a delay before relaxation of the system. Scattering can be observed with dilute solutions of milk. The protein polymers present in milk assume a globule-like conformation of the appropriate diameter to scatter visible light. You should observe dispersion of your beam throughout the sample as a result.
Oh, and if you want to impress someone at a party, you can fluoresce a gin and tonic.
The Shoe-Box Spectrometer Demo
- A white-light source
- A 3×5 inch note card
- A shoebox
- Flat-black spray paint
- Black duct tape
- Black card stock
- Hot glue
- A diffraction grating (A pair of children’s fun color glasses, the type you put on and look around at lights to see amazing radial distributions of rainbow, make suitable diffraction gratings)
- A prism
- Begin by painting the inside of your shoe box with flat-black spray paint. This coating prevents stray light from reflecting within the instrument.
- Construct a cube from card stock that is slightly shorter than the height of your shoe box. The width should be about 2-3 inches. The top of the cube should be free to open and close, and no bottom is required.
- On opposite sides of your card stock cube cut a very thin slit (about 2 mm) and a larger slit (about 5 mm).
- Using duct tape, attach the diffraction grating centered over the thin slit.
- Butt the card stock cube against the rear (long end) of the shoe box top. Be sure to leave room for the bottom to fit between the cube and the lid lip. Finally secure the cube to the lid with duct tape and glue.
- Place the shoe box lid down and lay the box on top. Make note of where the large slit of the sample chamber lays in conjunction with the shoe box. Cut a slit or hole that is about 10 mm in the shoe box directly behind the large slit. The decreasing slit widths will help to collimate your light source.
- Align your light source behind the entrance slit and turn it on. Light should pass into the sample holder through the large slit, and out through the thin slit/ diffraction grating. Turn off the lights and use your note card to get a feel for where the best image is produced after the light exits the diffraction grating. It may be necessary to place a prism between the light exiting the diffraction grating and your note card in order to help spread the spectral lines.
- Once you have found a suitable arrangement, tack down the prism and note card with hot glue. The final step is to cut a suitable viewing hole in your shoe box. This should be positioned so that you can clearly see the note card but minimal stray light can enter.
You should be able to use sample from the Light and Matter experiment above to test your instrument. First take note of the spectrum produced by your light source alone. It may be useful to place a number line on your note card to mark where certain spectral lines appear. Next, take a sample and place it with in your card stock sample chamber and observe the new spectrum produced. If your sample absorbs in the region of the electromagnetic spectrum produced by your light source you should see certain spectral lines removed from the spectrum produced on your note card.
What is going on here?
Using a polychromatic light source such as an incandescent light bulb produces all the colors of visible light that we are familiar with. As light passes through your diffraction grating it is dispersed into its components, which we can observe as the ROYGBIV rainbow on our note card. When we place a sample in the instrument we expect interaction between the sample and the incident light. For instance, if we place our solution of green dye (it should be noted that a fairly dilute sample is required) in the instrument, then we should see absorption of the red wavelengths present in our light source. As light exits the sample and is dispersed by the diffraction grating, we will no longer see lines in the red portion of our rainbow.
In general, what we have done is take the principles learned during the Light and Matter demonstration and build an instrument that exploits those mentioned phenomena to tell us something about the physical world. Everyday scientists use spectrometers of this type to identify unknown compounds in mixtures. In general, analytical instruments have a simple scheme, a stimulus, a sample holder, and a detector. While today’s computer driven instruments may seem intimidating, for the most part, they follow the above scheme. Our eyes make excellent detectors, so try to think of what other household instruments you can construct.
Teachers can use this simple setup with containers of house hold chemicals to show how they subtract from the spectrum of your incident light source.
If you play around with light sources, you quickly realize LED light sources don’t produce a full spectrum. You can ask students to recognize that light emission in LEDs must be due to a different mechanism than that in an incandescent light bulb. The slit has major effects on performance. Students are encouraged to start by trying to cut into a note card; they will see that the slit transmits a minuscule amount of light. Encourage them to try and make wider slits. Next, you might saw a slit in a small piece of wood. Finally, note that razor blades make immaculate slits (note, use proper safety when using the razor!).
It is amazing how much you can learn from empiricism. Trial and error teaches a student why cuvettes are square, why slits are made of metal, why the spectrometer is closed into a box, and why that box is black on the inside. Further, students start to appreciate how mathematics can be used to tune the distances between elements and focus your spectrum.
Exploration of science through hands-on experience reenforces learned theory and is lots of fun, too.
More on the Theory of Spectroscopy
Spectroscopy comes in many varieties, categorized by the type of light involved. In a spectroscopy experiment we primarily measure light before and after it interacts with a sample; this often leads to the name “spectrophotometry”. However, we can also make spectrophotometric measurements just by observing a sample that has been stimulated by another form of energy. In fact, the earliest spectrophotometric measurements by Kirchhoff and Bunsen were of this type; a metal was dissolved in solution or powdered and placed into a flame, where new colors were emitted in the flame and analyzed.
The underlying principle of spectroscopy is light’s ability to interact with matter. Light demonstrates wave-like properties with electric and magnetic fields that oscillate perpendicular to each other. Because matter is composed of atoms, which possess charged particles, there is an electric field present for light to influence. We observe this everyday through visible properties such as color, opaqueness, transparency, luster, and more. Each of these observed properties arises from a fundamental interaction of light and matter, including absorption, scattering, and emission.
Absorption: When an atom or molecule is struck by an incident photon, the energy of that photon is stored in the atom or molecule briefly. The energy of the incident photon, which is wavelength-dependent, determines the mechanism of storage.
Scattering: When light waves propagating through a medium interact with irregularities whose size is on the order of the propagating light waves’ wavelength, scattering occurs. Scattering can be divided into two categories: elastic and inelastic. In elastic scattering, an incident beam of light is caused to divert from its original path. An example of this is our blue sky; as white light from the sun hits our nitrogen-rich atmosphere, the nitrogen is effective at scattering the blue components of visible light. Because the blue light no longer follows a direct path through the atmosphere, we observe blue light throughout the sky. Further, as you look towards the horizon, you are looking through more atmosphere; blue light is scattered to an almost imperceptible amount, so at sunset we primarily see red light, which transverses the atmosphere largely unaffected.
In the inelastic case of scattering, the energy of the incident photon and the scattered photon differs. The change in energy can give the observer information on the motion or activity of the scattering centers.
The mechanisms that lead to scattering are out of the scope of this article but are worth looking into. It is important to note that absorption and emission are involved in producing a scattered beam of light. Scattering is more a resulting property of the bulk phase of a material than the result of a single photon interacting with a single atom or molecule.
Emission: When we have excited an atom or molecule to a higher energy state, it will at some point later relax back to its ground state (state of lowest energy). When the system relaxes, a photon of lower energy than the stimulus is produced.
Malcolm Davidson is a senior chemistry student at Louisiana State University. You can reach him at firstname.lastname@example.org for comments, suggestions, or just to talk shop. Photos courtesy of Jacob D. Mcalpin.
References and Further Reading:
Kirchhoff, Gustav, and Robert Bunsen. “Chemical Analysis by Observation of Spectra.”Annalen Der Physik Und Der Chemie 110 (1860): 161-89. Print.
Thomsen, Volker. “A Timeline of Atomic Spectroscopy.” Spectroscopy 21.10 (2006): n. pag. Spectroscopy Online.
Alexander Scheeline and Kathleen Kelley. “Cell Phone Spectrophotometer.” Cell Phone Spectrometer. N.p., n.d. 21 Nov. 2012. http://www.asdlib.org