Homework on spectroscopy, colors, and light. Answers should be entered on a Scantron form given out in class. This exercise is worth 20 points.

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1 Astr. 170B1 Due Sept. 20 Professors Rieke Homework on spectroscopy, colors, and light. Answers should be entered on a Scantron form given out in class. This exercise is worth 20 points. 1. Is the spectrum below a. an absorption line one b. a continuum c. an emission line one d. Doppler shifted e. unresolved 2. If a photon of energy E has a wavelength of 0.6 microns, what energy photon will have a wavelength of 0.3 microns? a. 2 times E b. Half of E c. 4 times E d. ¼ of E e. the energy cannot be determined accurately An energy level diagram showing the 3 hydrogen transitions which give the strongest lines in the visible spectrum. 3. The figure on the left illustrates electronic transitions from 3 higher levels to a lower one (5 2, 4 2, or 3 2) in the hydrogen atom. Which transition produces a photon with the highest energy? a. 5 -> 2 b. 4 -> 2 c.3 -> 2 4. Which transition produces a photon in the reddest portion of the spectrum? a. 5 -> 2 b. 4 -> 2 c. 3 -> 2 5. What type of spectrum (emission, or absorption) does the observer see in the figure on the right? a. absorption b. emission c. continuous 1

2 6. How are the electrons in the atoms in the hydrogen cloud in the figure above behaving? a. They are jumping from a lower to a higher level b. they are jumping from higher to lower c. They are leaving the cloud d. We cannot know how they are moving e. They are jumping from a lower to a higher level and then eventually falling back to the lower level 7. The young, hot star in picture to the left heats up the gas in the cloud. Assume that the observer can only see radiation emitted by the hydrogen cloud and cannot see the star. What type of spectrum (emission, or absorption) does the observer see? a. absorption b. emission c. continuous 8. How are the electrons in the atoms in the hydrogen cloud in this figure behaving? a. They are jumping from a lower to a higher level b. they are jumping from higher to lower c. They are leaving the cloud d. we cannot know how they are moving e. They are jumping from a lower to a higher level and then eventually falling back to the lower level 9. Look at the spectrum of the Sun below. What type of spectrum (continuous, emission line or absorption line spectrum) does the Sun produce? a. absorption b. emission c. continuous The next few problems will be much easier if you review how to use scientific notation (see for more practice) 2

3 10. Hydrogen has a spectral line that is observed at a wavelength of 656 nm (656x10-9 meters). Recalling that the speed of light is c = 3x10 8 meters/sec and the relationship between wavelength and frequency, frequency = c/wavelength, what is the frequency of the photons observed as this spectral line? a. 4.6x10 14 Hz b. 2.2x10-15 Hz c. 100 MHz d. 4.6 Hz e. Cannot be computed 11. The photons producing the hydrogen line in question 10 also have energy. Review the relationship between energy and frequency given in the lectures, energy = h times frequency. Use h=planck s constant = 6.6x10-34 Joules/Hz to compute the photon s energy which is Joules. a. 3 b. 3.0x10-19 c. 24 d. 24x10 19 e. 7x If you observed the hydrogen line discussed in questions 10 and 11, what color would it have? a. purple b. green c. white d. all the colors of the rainbow e. red A. B. Examine the two spectra above. Go to and compare the spectra shown there with these spectra. 13. The spectrum labeled A. above is produced by which element? a. lithium b. hydrogen c. helium d. carbon e. no way to tell 14. The spectrum labeled B. above is produced by which element? a. lithium b. hydrogen c. helium d. carbon e. no way to tell 3

4 A. B. C. D. 15. The spectra of four stars are shown above. Which star s spectrum is dominated by spectral lines from hydrogen? A. B. C. D. E. none are 4

5 16. The spectrum plotted above is from a real star, Vega. The spectrum is also shown in color as it might appear to your eye. What element s absorption lines are prominent in Vega s spectrum, particularly in the blue? a. lithium b. hydrogen c. helium d. carbon e. no way to tell The plot at the right shows a magnified section of the Vega spectrum in black. You may assume that Vega is not moving. The red plot is part of the spectrum from another star very similar to Vega except for its Doppler shift. The numbers show the location of the spectral line. Use c=300,000 km/sec 17. In what direction is the star producing the red spectrum moving? a. Toward us b. Away from us c. Cannot be figured out from this plot How fast is the star producing the red spectrum moving? a. 300,000 km/sec b. 150 km/sec c km/sec d km/sec e. 61,500 km/sec This figure is similar to the previous one but the blue plot is the spectrum from a third star with a different Doppler shift. In what direction is the star producing the blue spectrum moving? a. Toward us b. Away from us c. Cannot be figured out from this plot 5

6 flux 20. How fast is the star producing the blue spectrum moving? a. 300,000 km/sec b. 210 km/sec c km/sec d km/sec e. 86,160 km/sec 21. The graph below shows the output of a black body at a temperature of 5000K, against the wavelength. The peak is at 0.58 microns. How would the output against wavelength change if the temperature were 2500K? a. it would stay the same b. it would get fainter but otherwise stay the same c. the shape of the curve of output against wavelength would change d. the peak of the output would appear at 1.16 microns e. the peak of the output would appear at 0.29 microns wavelength (microns) 22. In the situation described in problem 21, with the temperature going down by a factor of two, how will the total output over the entire range of wavelengths (the luminosity) change? a. stay the same b. get 2 times smaller c. get 4 times smaller d. get 8 times smaller e. get 16 times smaller 6

7 23. How low would the temperature have to be for the peak of the output to be in the far infrared, at 100 microns? a. 250K b. near absolute zero c. 29K d. -57C e. None of the above 24. The temperature of the sun is about 5800K. At what wavelength does the output of the sun peak? a microns b microns c. somewhere in the red d. none of these For the following questions, you will need to refer to the spectra you measured in the discussion session. 25. Spectrum number 1 is d. none of those 26. Spectrum number 2 is d. none of those 27. Spectrum number 3 is d. none of those 28. Spectrum number 4 is d. none of those 7

8 29. Spectrum number 5 is d. none of those 30. The mystery gas is a. argon b. helium c. hydrogen d. xenon e. none of these 31. The fluorescent lights emit a. a continuous spectrum b. a line spectrum c. a spectrum with lines and continuum together 32. The incandescent light (light bulb) emits a. a continuous spectrum b. a line spectrum c. a spectrum with lines and continuum together 33. What gas is in the fluorescent lights? d. hydrogen e. argon The forms below are for you to draw in the spectra you observe in the discussion section and to turn in (paper copies). We have provided extra ones in case you need them. You should have six drawings; label the ones you want graded in the margin, and be sure to put your name on the forms you turn in (and staple them together): 1. helium lamp 2. neon lamp 3. mercury lamp 4. mystery lamp 5. fluorescent room light 6. incandescent light (light bulb) 8

9 Name: 9

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