AST 105 HW #3 Solution
Week of September 7th, 2015
Note: All Problems are from The Cosmic Perspective (6ed)
Chapter 5
Review Problems
2. What is a spectrum, and how do we see one?
Answer:
Spectrum is the complete spectrum of electromagnetic waves, which includes radio waves, visible light, and
x-rays. We can see the visible spectrum of light by passing light through a prism.
6. What is a photon? In what way is a photon like a particle? In what way is it like a wave?
Answer:
A photon is a particle of light. Unlike an ordinary wave, light has a smallest unit that cannot be subdivided.
However, light also has wavelike properties, such as having characteristic wavelengths and frequencies.
14. Describe the conditions that would cause us to see each of the three basic types of spectra. What do
we see in the Sun’s spectrum shown on the opening page of this chapter?
Answer:
A continuous spectrum is seen when we have a hot object emitting light across a broad range of
wavelengths. We see an emission line spectrum when we look at a cloud of thin gas because we see only the
wavelengths of light that correspond to the atomic transitions allowed in that gas. We see an absorption line
spectrum when we look at a hot object (like a star) through a thin cloud of gas. The continuous spectrum
loses photons of the specific frequencies that the atoms in the thin gas like to absorb. The spectrum at the
start of the chapter shows a continuous spectrum from the Sun's hot "surface." However, before the light
reaches us, it passes through the Sun's thin atmosphere and photons of certain specific wavelengths are
absorbed by the gas. This creates the dark lines in the spectrum.
18. Describe the Doppler Effect for light and what we can learn from it. What does it mean to say that
radio waves are blueshifted? Why does the Doppler Effect widen the spectral lines of rotating objects?
Answer:
The Doppler Effect is the change in frequency in light due to the source's motion toward or away from the
observer. When the source is coming toward us, the light we see has a shorter wavelength (higher
frequency), and we say that it is blueshifted. If the object is moving away from us, the light has a longer
wavelength than we would expect, and we say that it is redshifted. For rotating objects, we see part of the
object coming toward us and part of it moving away from us. The light from the part coming toward us is
blueshifted, and the light from the part going away is redshifted. If we can't see the different parts of the
object clearly, we will see the blueshifted, the redshifted, and the unshifted light all at once, making it look
like the lines have been broadened.
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Test Your Understanding
Each of the following statements makes some type of claim. Decide in each case whether the claim could be
evaluated scientifically or whether it falls into the realm of non-science. Explain clearly; not all of these have
definitive answers, so your explanation is more important than your chosen answer.
19. The walls of my room are transparent to radio waves.
Answer:
This statement makes sense. Radio waves can travel through the walls of a room. That is why we can listen
to radio inside buildings.
20. Because of their higher frequency, X rays must travel through space faster than radio waves.
Answer:
This statement does not make sense. All the spectrum of light travel with the same speed, 3*10^8 m/s.
21. If you could see infrared light, you would see a glow from the backs of your eyelids when you closed
your eyes.
Answer:
This statement does make sense. Your eyelids are warm, which means they emit infrared light.
22. If you had X-ray vision, then you could read this entire book without turning any pages.
Answer:
This statement does not make sense. Books are transparent to x-ray
25. If the Sun's surface became much hotter (while the Sun's size remained the same), the Sun would emit
more ultraviolet light but less visible light than it currently emits.
Answer:
This statement does not make sense. If the Sun’s surface becomes much hotter, the ratio between the
amount of ultraviolet light and visible light will change. But it will emit more of both ultraviolet light and
visible light.
Quick Quiz
Choose the best answer to each of the following. Explain your reasoning with one or more complete
sentences.
29. Why is a sunflower yellow? (a) It emits yellow light. (b) It absorbs yellow light. (c) It reflects yellow
light.
Answer:
(a) The color of an object is portion of visible light reflected off its surface.
35. If you heat a rock until it glows, its spectrum will be (a) a thermal radiation spectrum. (b) an
absorption line spectrum. (c) an emission line spectrum
Answer:
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(a) If a large and dense object, such rock, gets heated, it emits a thermal radiation spectrum.
36. The set of spectral lines that we see in a star’s spectrum depends on the star’s (a) atomic structure. (b)
chemical composition. (c) rotation rate.
Answer:
(b) Spectral lines are caused by emission or absorption of light by atoms or molecules. Therefore, the set of
spectral lines in a star’s spectrum tells what the star is composed of.
Short-Answer/Essay Questions
44. Energy Level Transitions. The following labeled transitions represent an electron moving between energy
levels in hydrogen. Answer each of the following questions and explain your answers.
a. Which transition could represent an atom that absorbs a photon with 10.2 eV of energy?
Answer:
(b) When a hydrogen atom absorbs a photon with specific energy, its electron will go up level(s) if the
difference between the energy of the initial state and final state is equal to the energy of the photon
absorbed.
b. Which transition could represent an atom that emits a photon with 10.2 eV of energy?
Answer:
(c) It is exactly the opposite of what happened in part (a)
c. Which transition represents an electron that is breaking free of the atom?
(e) If an electron attains enough energy to go over ionization level, it breaks free of the atom.
d. Which transition, as shown, is not possible?
Answer:
(d) An electron can go up (or down) only full level(s). For example, it can’t go from level 1 to level 1.5.
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e. Would transition A represent emission or absorption of light? How would the wavelength of the
emitted or absorbed photon compare to that of the photon involved in transition C? Explain.
Answer:
Transition A represents an electron falling from level 3 to level 1, emitting 12.1 eV of energy in the process.
This is less wavelength than in transition C.
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