EXPERIMENT 09 OBSERVATION OF SPECTRA INTRODUCTION: In physics, as in very other area of study, one of the most valuable questions a student can learn to ask is, How do they know that? Thus, when you read that Wolf- 457 has a density of 20.000 tons/in 3, or the universe is 18 billion years old, or that a gravitational black hole has been observed, you should always stop to ask the question, How? Practically everything we know about universe beyond the earth results from an analysis of the spectrum of the light coming to us. Likewise, the spectrometer is one of the most powerful instruments available to the scientist and engineer in studding laboratory phenomena. THEORY: Two instruments are available for the purpose of forming a spectrum of visible light. The first, a prism spectrometer, consists of (1) a collimator which forms a parallel beam of the light radiated from a source, (2) a prism which uses the phenomenon of refraction to separate the light into its various components (colors), and (3) a telescope to enhance viewing of the spectrum. Figure 1: Prism - Spectroscope The second device is similar to the prism spectroscope except that a grating replaces the prism and produces a spectrum through the phenomenon of diffraction. The two general classes of spectra are emission (a series of bright lines or bands of color against a dark background) and absorption (a series of dark lines or bands against a bright background) spectra. There are three types of emission spectra: continuous, bright line, and band spectra. Continuous spectra, looking like a cross section of the rainbow, are usually produced by incandescent solids and liquids. Generally, bright line spectra are atomic spectra produced by incandescent gases and vapors as electrons make downward transitions from higher, more energetic orbits to the ground state. Band
spectra ordinarily have an origin which may be traced to molecular effects. An absorption spectrum is just what its name implies -- a spectrum in which some parts are missing are due to part of the light energy having been absorbed by passage through an absorbing medium. It may be of the dark band or the dark line type. The light filters used in photography produce absorption bands. Almost every liquid has an absorption band somewhere within the spectrum of visible light. The Fraunhofer lines of the sun s spectrum are the most noted example of dark line spectrum. THE EXPERIMENT: 1. APPARATUS: In this experiment you will use a Spectroscope (a replica diffraction grating) to observe the spectrum of the different discharge tubes. The tubes are powered with a high power voltage supply. All of these components are provided in your laboratory station. 2. PROCEDURE: 2.1 The Dispersion Curve Dispersion is the process of separating light into various wavelengths. This is done, primarily, with the use of a prism or grating. In the case of a prism, the light is divided according to its wave lengths because the glass has a different index of refraction for each color, the index becoming larger as the wavelength gets smaller. This means that when light passes through a prism, the red end of the spectrum is refracted less than the blue end of the spectrum. If one plots a curve showing the amount of deviation for each wavelength of light, he will obtain a dispersion curve for that particular instrument. Having plotted such a curve, it may be used to determine unknown wavelengths when their deviations are measured. Each curve is characteristic of the instrument for which it was drawn and cannot be used for any other instrument. The accuracy with which unknown wavelengths can be determined depends largely on the instrument for which the dispersion curve is drawn. Your dispersion curve will be quite crude. It will be only a few inches in length. Accurate curves, sometimes several feet in length, are useful in rapid spectrum analysis. Illuminate the slit of the spectroscope with the provided spectrum tube light. The tube containing the scale should be pointed toward a window or an artificial light. IT SHOULD BE UNNECESSARY TO DISTURB THIS TUBE. Looking into the telescope, focus it until a clear image of the slit is seen. IF THE SCALE IS NOT VISIBLE AT THIS TIME, HAVE AN INSTRUCTOR MAKE PROPER ADJUSTMENT FOR YOU. Focus the telescope until the yellow doublet is quite distinct. Read and record the positions of these two lines on the scale. Do likewise for other lines listed on the card at your lab station. Replace the mercury lamp with the other tube provided by your instructor and take the scale readings of the spectral lines listed on the card. Record the scale reading for three or four other lines not listed.
Go to the bulletin board and study the chart of spectra posted there-- this will aid you in identifying the mercury lines. Use the space provided on the worksheet to record the information requested. Use the graph paper on the worksheet to plot wavelength versus your scale reading; i.e., make a singe dispersion curve using the provided tubes spectra. (b) OBSERVATION OF SPECTRA For the remainder of this experiment you are to observe and chart the spectra of various sources on the worksheet. Each chart should be one cm high and each division on the horizontal scale is to represent one of the numbered units on the scale of the spectroscope. Place the wavelength in Angstrom units at the top of the charts. Use the colored pencils provided and see that they are returned at the end of the period. CAUTION: You can get a nasty shock from the high voltage required to excite the spectral tubes. BE SURE ALL POWER IS OFF BEFORE HANDLING TUBES IN ANY WAY. QUESTIONS: 1. The Doppler effect in sound manifests as a rise or fall in pitch (i.e., an increase or decrease in frequency) as a sound source approaches or recedes from an observer. How could be Doppler effect be observed in the light from a star and what kind of information can be gained from a study of the effect? 2. Fraunhofer lines are dark lines on the continuous spectrum of the sun. What is the origin of these lines? How might one determine something about the atmosphere of the Mars by observing sunlight reflected from the planet? 3. A board beam of light from a sodium vapor lamp flame of a Bunsen burner. A small screen is placed on the other side of the burner. Salt is sprinkled into the flame and brilliant yellow light is observed. What is the origin of this light? When salt is sprinkled into the flame, a shadow of the flame appears on the screen. In the absence of salt, no shadow is seen. Explain.
AUI PHY 1402 LAB. REPORT EXPERIMENT 09 OBSERVATION OF SPECTRA NAME:.. DATE:.. SECTION:.. * * * 1. EXPERIMENTAL PURPOSE: State the purpose of the experiment.( 5 points )
2. EXPERIMENTAL PROCEDURES AND APPARATUS: Briefly outline the apparatus used and the general procedures adopted. (5 points )
3. RESULTS AND ANALYSIS TABLE 1: (25 points) SPECTRAL LINES OF: SPECTRAL LINES OF: wavelength scale reading wavelength scale reading GRAPH (25 points)
SPECTRA (25 points)
4. CONCLUSIONS: (5 points) QUESTIONS: (10 points)