Spectroscopy of Various Light Sources: The Interactions between Light and Matter ASTR 170B1, Spring 2017, Lab #2 DUE IN CLASS ON Thursday Sept 28! You CAN work in a group of 2, but you need only turn in one copy for your group. The answers must be neatly and legibly written in the space provided and spectra neatly drawn and labeled so we can understand what you wrote. Any additional materials we provide should be attached to the back of the lab. Names of the members of your group: 1 Introduction Although we usually think of objects as having a single color -- such as the red, yellow and green lamps of a traffic light -- most of the light that we see is actually composed of radiation at many different wavelengths. For example, the light from an ordinary light bulb appears to be white or yellow. In fact, a light bulb emits light of all colors, from red to yellow to blue, sometimes at different intensities. The human eye senses light of different colors simultaneously. Just as a painter mixes red paint with yellow paint to get orange paint, your eye adds up all the colors of the light emitted by the bulb to form a single visual impression of color or hue. A spectroscope does the opposite. Instead of adding up all the light at different wavelengths to arrive at a single color, a spectroscope takes light and splits it up into its constituent wavelengths. Then the intensity of light at each wavelength can be measured, either by eye or with an electronic detector. 2 Method This lab has two parts. You will measure the spectra of various light sources in the first part using your eye and a handheld spectroscope. In the second part, we will all collectively observe spectra using a digital spectroscope. Some of these parts involve analysis that you will do out of class. Throughout this lab, you'll use Kirchoff's Laws and the Bohr model of the atom to explain what you saw. You'll have to start by learning to use our spectroscopes. Since the grating disperses the light, you need to learn how to Page 1
look through the spectroscope correctly. We'll demonstrate and help you. There are calibrated wavelengths superimposed on the spectroscope itself. You can then identify colors by quoting approximate wavelengths. Remember, colors and wavelengths are really the same thing, as is energy of the photons involved. There are descriptions and questions here to help guide you as you use your time to learn about the spectroscopy of various sources. 3 Observations of Different Types of Spectra In the following parts, we will ask you to use the templates attached at the end of this lab to make sketches of the spectra that you observe through the spectroscope, using a pen (black or blue ink only, please!) or a pencil. If you happen to have a set of colored pencils or crayons with you, and you re feeling fancy, then feel free to use them. In any case, please annotate your sketches, noting anything you see in the spectrum, including, but not limited to, the range of wavelengths/colors/energies that you see, and any noticeable emission or absorption lines! In the spaces we provide, write your answers in complete sentences. 4 Incandescent Light Bulbs Examine the spectra the incandescent light bulb sources. Each of these are basically a very hot wire. On the left and right sides of the spectrum, the colors appear to fade to black. An incandescent light bulb does radiate energy in these wavelength ranges, but you would need specialized equipment to detect it. Question 1: Look at the incandescent bulbs (the overhead projector) with your spectroscope. What do you see? Sketch the spectrum on a template on the last pages of this packet, and describe what you see in the space below. Label the spectrum and note the wavelengths of the ends of the spectrum, that is, the longest and shortest wavelengths you can see. Page 2
Question 2: Is the spectrum you see a continuous spectrum, an emission spectrum, or an absorption spectrum? Why do you think this is the case? 5 Fluorescent Light Bulbs A fluorescent light bulb is a painted glass tube filled with gas. To light the tube, electrical energy is injected into the gas by applying a high voltage inside the tube. The addition of electrical energy causes the electrons in each atom to jump to a higher energy level. After being excited to a higher energy level, the electron will drop back to a lower energy level and emit a photon. Using your spectroscope, carefully examine the spectrum of a fluorescent light. Use the fluorescent ceiling lights in this room to examine this kind of spectrum. Question 3: What do you see through your spectroscope? As before, draw the spectrum on a template, and use the space below to describe what you see. Be sure to label the template. Page 3
Question 4: Is the spectrum you see a continuous spectrum, an emission spectrum, or an absorption spectrum? Why is this? 6 Light Emitting Diodes An LED is a semiconductor device, very similar to the billions of transistors that make up every modern computer. Applying electricity creates excited electrons that radiate light as they lose energy. Each color of LED is determined by the specific properties of the material it is made from. We will show you three LEDs, a red one, a blue one, and a white one. Please sketch what you see for each. Question 5: What do you think you have to do to make white light? Page 4
Question 6: Sketch the spectrum of the white LED. In words, compare it to the spectrum of the incandescent source. 7 Fingerprints of the Elements Discharge tubes are very similar to fluorescent light bulbs. They apply a current to a sample of gas inside a glass tube, and the gas discharges the electrical energy in the form of light. We can use this to examine the spectrum of samples of pure gases. In this part of the lab activity, we will use discharge tubes to examine the spectra of hydrogen (H), helium (He), neon (Ne), and one other elemental gas. You will sketch the spectrum of each kind of gas. Sketch these. Be really careful because you ll need these later. Question 7: Look at the spectra from the discharge tubes. What do you see? Draw the spectra on the templates provided, and label them. Describe the spectra below, including any similarities and differences among them. Page 5
Question 8: How would you describe these spectra: continuous, emission, or absorption? Why is this? Page 6
Question 9: How do the discharge tubes relate to the Bohr model of the atom (which states that electrons can only be in very particular energy states)? 8 Further Questions We will start class by showing you digital spectra of some gases and will provide those spectra on the website (Lab area). Once you have completed the observations in the previous sections, then continue on to these questions. Question 10: Do your best to redraw the diagram you drew of the fluorescent bulb in the same style as the graph on the computer, plotting brightness (as best you can tell) versus color/wavelength. Question 11: Based on what you saw in the fluorescent bulb, and discharge tube spectra, what similarities did you notice? What differences do you notice? What do you think is causing these similarities and differences? Page 7
Question 12: Comparing your observations of the hydrogen spectrum to the spectrum of one of the other gases (like neon), what do you notice that seems distinct about the hydrogen spectrum? Why do you think this is? Recall that hydrogen is a very simple atom, with only one electron bound around one proton. How many protons and electrons do the atoms in the other gas have? How do you think this affects the number and complexity of possible energy levels for those electrons to exist in? The Solar Spectrum To the right is a high resolution spectrum of the sun. Many modern digital spectrographs are sensitive to light that you cannot detect with your eyes. The Sun, like pretty much every other star, can be thought of as a hot, dense ball of gas, surrounded by a layer of cooler, less-dense gas. The inner part emits a continuous spectrum, like the filament of the incandescent light bulb or a glowing hot iron fire poker, and the outer layer of gas absorbs some of that light. Any elements in that gas will leave their unique fingerprint in the light that eventually reaches us on Earth. From this, we can deduce how much of what kinds of atoms are in the Sun! We will show you the sky using our digital spectrometer. How does it compare to this higher-resolution spectrum? Page 8
Question(s) 13: Describe the shape, and any notable features that you see in the solar spectrum. What is the wavelength range of the graph? At what wavelength does the Sun s spectrum appear to give off the most amount of light? At what wavelengths are the most obvious absorption lines? What about the smaller/less prominent lines? Do all the lines have the same width? The same depth? Question 14: Here you see the spectrum of a very Sun-like star, with some annotated features from hydrogen (H alpha and beta ), calcium (Ca H+K ), sodium (Na D ), magnesium (Mg b ), and calcium hydride (CH). Note that 10 Angstrom is the same as 1 nanometer (nm), so that 500 nm is the same as 5000 Angstrom. Are there any differences between our spectrum of the Sun, and this other spectrum? What do you think these differences are due to? What about the similarities between the two spectra? Page 9
Question(s) 15: Using Kirchoff s Laws, explain WHY the sun has BOTH a continuous spectrum AND absorption lines. Page 10
Question(s) 16: The digital spectra of the discharge tubes are unlabeled. There are 5, four of which are the H, He, Ne, Hg that you saw today, and one of which is new to you. Your job is to identify the correct 4 with correct identification (and you need to tell us how you came to your conclusion), and to tell me which one you didn't see, based on comparison with your hand-drawn spectra. Page 11
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