Experiment 24: Spectroscopy Figure 24.1: Spectroscopy EQUIPMENT High Voltage Power Supply Incandescent Light Source (3) Gas Discharge Tubes: 1. Helium 2. Hydrogen 3. Unknown Element Spectrometer Felt (1) Flashlight per person (2) Plastic Baskets (Elevate Light Sources) Oven Mitt Spectra Chart Figure 24.2: To rotate spectrometer, turn the column, not the pointer. 139
140 Experiment 24: Spectroscopy Advance Reading Text: Di raction grating, atomic spectra, line spectra, continuous spectra, Balmer series, electromagnetic radiation, visible light, absorption spectra, emission spectra. Objective The objective of this experiment is to investigate continuous spectra qualitatively and line spectra quantitatively. The Balmer series of hydrogen will be measured, then calculated. Theory Electrons orbiting an atom can be excited by raising them to a higher orbital. When the electrons return to a lower energy level, energy is released in the form of electromagnetic radiation, i.e., light. When electrons emit frequencies of light between 4 10 14 Hz (red) and 7 10 14 Hz (violet), the energy emitted can be observed by the human eye. This is the visible spectrum. The lowest frequencies of visible light appear red, then orange, yellow, green, blue, and violet follow as the frequency increases. As the frequency increases, so does the energy, since energy is proportional to frequency. The visible spectrum is a narrow range of the electromagnetic spectrum: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. Continuous Spectra Liquids and solids must be hot for the electrons to emit visible light. The atoms and molecules interact and emit all frequencies of light in a continuous spectrum. The relative quantities of emitted frequencies depend on the temperature of the liquid or solid. For example, when an electric stove element is about 500 C, the element begins to emit visible light and glow red hot. When an object nears 1500 C, all visible frequencies are noticeable; the object appears white hot. The most common source of artificial white light is the incandescent light bulb. The Sun is our most common source of natural white light. Grating Spectrometer The grating spectrometer is a device that allows one to calculate the wavelengths of light emitted from a light source. The device consists of a di raction grating attached to a collimator (small aperture resulting in parallel rays of light). The di raction grating (many slits instead of one or two) divides the light up into its spectral components. The telescope allows the spectrum to be observed. The bottom of the spectrometer is ruled for angular measurements. If the grating spacing (the number of grooves in a given distance) is known, the wavelength of the light can be found from Eq. 23.3 as follows: Line Spectra sin = m d (24.1) Gases emit line spectra when a high voltage is applied to the gas at low pressure. Electric current excites electrons, which drop between discrete energy levels and release quantities of energy predetermined by the orbital structure of the atom. The spectrum of energies released appears as a series of single frequencies, each a di erent color. Line spectra are unique for each element and can be used to identify elements. where m is the order number (m = 1 for this experiment), is the wavelength of that particular spectrum line, and d is the grating spacing (d =1.67 10 6 m). Some elements allow for viewing of a small part of the second order spectrum, m = 2. Higher order spectra are a repetition of the spectrum at a greater di raction angle.
Prelab 24: Spectroscopy 141 Name: 1. Define continuous spectrum and line spectrum. (10 pts) 2. What is the range of frequencies of visible light? What is the range of wavelengths of visible light? (20 pts) 3. Using Eq. 24.1 and the grating spacing given in the lab procedure, calculate the wavelength of a spectral line assuming that is equal to 13.86 and the order number is one (i.e., m = 1). What is the color of this line? (20 pts) 4. Where is the second order line of the spectral line in Question 3? Calculate for this wavelength while m = 2. (20 pts) 5. What are the first three wavelengths of the Balmer series? Show work. [Note: Eq. 24.2 requires that n 3] (30 pts)
142 Experiment 24: Spectroscopy Warning: The power supply operates at 5000 V. Use caution! The power supply must be turned o and unplugged before discharge tubes are inserted or removed. The discharge tubes will get hot! After obtaining the necessary values, turn o and unplug the power supply, lay it flat on the table, and place the cord on the table. This allows the tube to cool while you perform calculations. Hold the end of the tube when handling, not the center. The collimator must be positioned close to, but not touching, a light source. It must focus on the wire inside the incandescent light bulb or the center of a discharge tube (Fig. 24.1). PROCEDURE PART 1: Continuous Spectrum 1. Position the incandescent bulb on the wood block or other platform. Align the spectrometer with the bulb, a few centimeters away. 2. Align the telescope near zero degrees and turn on the light bulb. Adjust the telescope until the white light from the filament is visible and focused. Record the angle of this central band. All angle measurements will be made relative to this position. 3. Rotate the telescope slowly to the left from 0 out to 30. Sketch the bands of color your observe. You should see a mirror image of the spectrum to the right of 0 ; check all your angle measurements against this spectrum. 4. Mark the degree at which the spectrum fades completely at either end. Calculate the wavelength of light at these positions using Eq. 24.1 (for this experiment: m = 1, d =1.67 10 6 m). Show work. 5. Are these wavelengths comparable to the accepted extreme wavelengths of the visible spectrum? Calculate your percent error from the values 750 nm (red) and 400 nm (violet). PART 2: Line Spectra Known gas: Helium 6. Place the high voltage power supply on the platform. Insert a helium spectrum tube into the power supply and align the spectrometer with the tube. 7. Turn on the power supply and locate the central band of light near zero degrees. 8. Rotate the telescope to the left and sketch the line spectrum visible to you. Check your measurements with the mirrored image to the right of 0. 9. Check that this matches the known spectrum of helium. 10. Mark the location of the bright yellow/orange band; record its angle. Calculate the wavelength of this band using the equation mentioned above. 11. Compare this to its known wavelength, 587.6 nm. Balmer Series 12. Arrange the hydrogen discharge tube and measure for all spectral lines that are visible to you. 13. Calculate for each line (Eq. 24.1). 14. Calculate the wavelengths from the Balmer Series: 1 1 1 = R 2 2 n 2 (24.2) where n =3, 4, 5,...and R is the Rydberg constant (R =1.097 10 7 m -1 ). Unknown gas: 15. Replace the helium spectrum tube with the unknown tube. Observe the emission spectrum of this element, making any necessary sketches. 16. What element is this? Refer to the chart provided.
Experiment 24: Spectroscopy 143 QUESTIONS 1. What are some of the major sources of uncertainty in this experiment? 2. A red car parked under a mercury vapor lamp (similar to a discharge tube) does not look red. Why? (Refer to the line spectra chart.) 3. If you were to look at a fluorescent light with a spectrometer, which type of spectrum would you see? (You might want to observe this while you are still in the lab with the spectrometer. You may take your spectrometer or di raction grating to the hallway, the tutoring room,... ) 4. Compare each of your experimental hydrogen line spectrum wavelengths to the Balmer Series wavelengths. 5. Voltage does not kill, so why must you be careful around high voltage? (Refer to Experiment 15: Ohm s Law.)