SPECTROSCOPY SYNOPSIS: In this lab you will eplore different types of emission spectra, calibrate a spectrometer using the spectrum of a known element, and use your calibration to identify an unknown element. You will also observe the spectrum of the Sun using the observatory's heliostat. EQUIPMENT: Hand-held spectroscope, grating spectrometer, spectrum tube power supply and stand, helium, neon, nitrogen, and "unknown" spectrum tubes, incandescent lamp, heliostat, pencil. WARNING: There is high voltage on the spectrum tube. You can get a nasty shock if you touch the ends of the tube while it is on. Also, the tubes get hot and you can burn your fingers trying to change tubes. Let the tubes cool or use paper towels to handle them. Most of what astronomers know about stars, galaies, nebulae, and planetary atmospheres comes from spectroscopy, the study of the light emitted by such objects. Spectroscopy is used to identify the composition, temperatures, velocities, pressures, and magnetic fields of astronomical bodies. An atom emits energy when an electron jumps from a high-energy orbit around the nucleus to a smaller low-energy orbit. The energy appears as a photon of light having an energy eactly equal to the difference in the energies of the two electron levels. A photon is a wave of electromagnetic radiation whose wave-length (distance from one wavecrest to the net) is inversely proportional to its energy: high-energy photons have short wavelengths, low-energy photons have long wavelengths. Since each element has a different electron structure, each element emits a unique spectrum of light. Electron Small Electron Transition Low Energy, Long Wavelength Photon (Red) Nucleus Allowed Electron Orbits Medium Electron Transition Large Electron Transition Medium Energy, Medium Wavelength Photon (Green) High Energy, Short Wavelength Photon (Violet) The human eye perceives different energies (wavelengths) of visible light as different colors. The highest energy (shortest wavelength) photon
detectable by the human eye has a wavelength of about 4000 Angstroms (one Angstrom equals 10-10 meters) and is perceived as "violet". The lowest energy (longest wavelength) photon the eye can detect has a wavelength of about 7000 Angstroms, and appears as "red". A spectroscope is a device which allows you to view a spectrum. Light enters the spectroscope through a slit and strikes a grating (or prism) which disperses the light into its component colors (wavelengths, energies). Each color forms its own separate image of the opening; a slit is used merely to produce narrow images, so that adjacent images do not overlap. Spectrum Red Green Violet Hand-Held Spectroscope Grating Eye Source of Light Slit A grating is a sheet of material with thousands of evenly spaced parallel openings. In general, the crests and troughs of the light waves passing through adjacent openings destructively interfere with each other: they cancel each other (crest on trough) so that no wave survives. At a unique angle for each wave-length, however, the waves constructively interfere (crests add to crests, troughs to troughs); at that angle we see a color image formed by that wavelength of light. Longer wavelengths of light are "bent" (diffracted ) the most, so a spectrum is formed with violet seen at small angles, and red at large angles. Part I. Continuum and Emission Line Spectra I.1 Install the helium discharge tube in the power supply and turn it on. Describe the apparent color of the glowing helium gas. Now view the gas using the hand-held spectroscope, and note the distinctly separate colors (spectral lines ) of light emitted by the helium atom. Make a sketch of the spectrum and label the colors. Compare the apparent color of the glowing gas with the actual colors contained in its spectrum. I.2 Turn off the helium tube and replace with the neon gas tube. Judging from the appearance of glowing neon, what wavelengths would you epect to dominate its spectrum? Observe and sketch the spectrum using the spectroscope as before. Judging from the number of visible energy-level transitions (lines) in the neon gas,
which element would you conclude has the more comple atomic structure: helium or neon? A number of neon spectral lines are so close together that they are difficult to separate or resolve. This blending of lines becomes even more dramatic in molecules of gas: the constituent atoms interact with each other to increase the number of possible electron states, which allows more energy transitions, which in turn produces wide molecular emission bands of color. I.3 Install the nitrogen gas tube containing diatomic (two-atom) nitrogen molecules. Observe and sketch the spectrum, and identify the broad emission bands caused by its molecular state. A solid glowing object such as the tungsten filament of an incandescent lamp will not show a characteristic atomic spectrum, since the atoms are not free to act independently of each other. Instead, solid objects produce a continuum spectrum of light regardless of composition; that is, all wavelengths of light are emitted rather than certain specific colors. I.4 Look through the spectroscope at the incandescent lamp provided by your lab instructor. Sketch the spectrum, and confirm that no discrete spectral lines are present. Fluorescent lamps operate by passing electric current through a gas in the tube, which glows with its characteristic spectrum. A portion of that light is then absorbed by the solid material lining the tube, causing the solid to glow or fluoresce in turn. I.5 Point your spectroscope at the ceiling fluorescent lights, and sketch the fluorescent lamp spectrum. Identify which components of the spectrum originate from the gas, and which from the solid. Part II. The Helium Spectrum A spectrometer is a more sophisticated form of spectroscope used to measure the angle at which light is diffracted, and therefore enables the user to measure the actual wavelengths in the spectrum.
30 20 Collimator Telescope 10 Pointer 0 Light Source Slit Lens Grating Lens GRATING SPECTROMETER Focus Knob (B) Eyepiece (A) 20 10 30 The spectrometer consists of a collimator, which focusses the light from the slit onto the grating, and a telescope which permits viewing of a portion of the spectrum at a specific angle. A crosshair in the eyepiece is used to center the telescope on a spectral line; the angle is read from a scale below the eyepiece. Two spectra are formed, one on each side of the central (undiffracted) image of the slit. Install the helium spectrum tube in the holder and turn it on. Position the tube directly in front of the collimator slit, and align the telescope with the collimator (at 0 angle). Looking through the eyepiece, find the vertical stripe of light that is the image of the slit. Shift the position of the spectrum tube to one side or the other until the line is brightest. Adjust the telescope focus as needed: First, hold the focus ring (B) fied while sliding the eyepiece (A) forwards or back until the crosshair comes into sharp focus. Second, rotate the eyepiece (A) until one of the crosshair lines is vertical. Third, focus the telescope by moving the focus ring (B) forwards or back until the slit appears sharp and narrow. Check for proper spectroscope alignment: Verify that the angle pointer reads approimately 0 and that the slit appears vertical. Swing the telescope to the left and find the following spectral lines: violet, blue, blue-green, green, yellow and red (you may also see several additional fainter lines). If the lines are dim, move the spectrum tube to the left or right to make them brighter. Swing the telescope to the right and find the lines again. Ask your lab instructor for assistance if you have difficulties. II.1 Measure the diffraction angle of each line you can detect in the helium spectrum. Align the crosshair with each spectral line on the left side of zero, and read the angle from the pointer (the scale is marked at 0.5 intervals; estimate your measurement to the nearest 0.1 ). Enter your angles in the table below. Repeat the procedure with the spectrum on the right side, and average the two measurements of each line for greater accuracy. Each lab partner should make his or her own measurements.
HELIUM SPECTRUM DIFFRACTION ANGLE (Degrees) Wavelength (Angstroms) Left Side Right Side Average 7065 Deep Red (dim) 6678 Red (bright) 5876 Yellow (very bright) 5048 Green (very faint) 5015 Green (bright) 4922 Blue-Green 4713 Blue 4471 Violet (bright) 4388 Violet (dim) 4026 Deep Violet (very faint) II.2 Construct a calibration graph showing the relationship between wavelength and diffraction angle for your spectrometer. Use Angstrom wavelength units for the vertical ais, and angular degree units for the horizontal ais. Plot your data of wavelength versus average diffraction angle, and draw a best-fit straight line through the points as shown on the net page. 7000 Wavelength (Angstroms) 6000 5000 4000 Angstroms Degrees 14 16 18 20 22 24 26 Diffraction Angle (Degrees)
7000 6500 WAVELENGTH (ANGSTROMS) 6000 5500 5000 4500 4000 12 14 16 18 20 22 24 26 28 DIFFRACTION ANGLE (DEGREES) Part III. Identifying an Unknown Gas Select one of the unmarked tubes of gas (either hydrogen, mercury, or krypton). III.1 Install your "mystery gas" in the holder, adjust its position to maimize brightness in the spectrometer, and measure the positions of the spectral lines on both the left and on the right sides as before. Average the two measurements for each line. If your unknown has more than five lines, pick the five brightest. UNKNOWN SPECTRUM DIFFRACTION ANGLE (Degrees) WAVE - LENGTH Line Color, Brightness Left Side Right Side Average (Angstroms )
III.2 III.3 Use your calibration graph to convert your degree measurements of each line into wavelength (find the degree measurement on the - ais; go up to your line; then go left to the y-ais and find the wavelength). Identify the composition of the gas in the tube by comparing your measured wavelengths with the known wavelengths of light emitted by the gasses (given in Angstrom units in the tables below). Hydrogen 6563 Red 4861 Blue-Green 4340 Violet 4101 Deep Violet (dim) Mercury 5790 Yellow 5770 Yellow 5461 Green 4916 Blue-Green (dim) 4358 Violet Krypton 6458 Red 6427 Red 5871 Yellow-Orange 5570 Green 4502 Violet 4459 Violet 4368 Violet 4320 Violet 4274 Violet Part IV. The Solar Spectrum If it is sunny outside, your lab instructor will set up the Observatory heliostat to display the spectrum of the Sun. Recall that when you pointed your spectroscope at a fluorescent lamp, you observed a bright emission spectrum coming from the hot, glowing interior gas of the tube. Superimposed on that was a continuum spectrum coming from the cooler solid material lining the eterior of the tube. This situation is reversed with sunlight: the very hot, very dense gasses in the interior layers of the Sun behave as a solid, emitting a continuum spectrum. The cooler, rarified atoms in the eterior layers of the Sun are free to act
independently, and so produce the characteristic line spectrum of the atomic species. The line spectrum is an absorption spectrum rather than an emission spectrum, however, as the atoms "steal" energy coming from below at very specific wavelengths. An absorption spectrum is produced by the opposite phenomenon as an emission spectrum: an incident photon of light is absorbed by an atom and provides the energy for an electron to jump from a low-energy orbit to a high-energy orbit. The energy of the photon must be eactly equal to the difference in the energies of the two levels; if the energy is too much (wavelength too short) or too little (wavelength too long), the atom can't absorb it and the photon will pass by the atom unaffected. Thus, the atom can selectively absorb light from a continuum source, but only at the very same wavelengths (energies) as it is capable of emitting light. An absorption spectrum appears as dark lines superimposed against a bright continuum of light, indicating that at certain wavelengths the photons are being absorbed by overlying atoms and do not survive passage through the rarified gas. IV.1 IV.2 Where in the solar spectrum is the continuum emission the brightest? Use the Fraunhofer Solar Spectrum Chart to identify the name (letter) assigned to a dark absorption line. Use the associated table to identify its wavelength and which atomic element is responsible for it. Even though your eye can't detect it, the solar spectrum etends beyond the visible red portion of the spectrum to include infrared light, and beyond the violet to include ultraviolet light. The spectrum of ultraviolet light can be made visible by fluorescence (remember the solid material in the overhead tube lights?). IV.3 Hold a piece of bleached white paper up to the viewing screen where ultraviolet light should be present in the spectrum. Note that there are many additional absorption lines in the solar spectrum at wavelengths that are not normally visible to us. Use the Solar Spectrum Chart to identify the names, wavelengths, and element responsible for the two etremely broad, dark lines in the spectrum. Your lab instructor will dim the solar spectrum by narrowing the entrance slit, and switch on comparison lamps containing hydrogen and neon; their emission spectra will appear above and below the solar spectrum. If an element is present in the Sun (and if the temperature of the gas is "just right"), its absorption line will appear at the same wavelength as the corresponding emission from the comparison source.
IV.4 IV.5 Identify which comparison spectrum (top or bottom) is due to hydrogen, and which is neon. Do you see evidence in the solar spectrum that hydrogen gas is present in the Sun? What about neon?