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Ay 1 – Homework #2 Distributed on April 8, due by 5 pm on Friday, April 15, to your head TA’s mailbox in rm. 249 Cahill, or hand it to your own Section’s TA Your name: Your section: Collaboration policy: you can discuss it among yourselves in general terms, but everyone has to write or derive their own solutions. Multiple choice questions have only one correct answer, worth 1 point; circle only one. 1. Compared to a cold (low T) blackbody, a hot (high T) blackbody
A.
B.
C.
D.
E.
Emits more photons per unit time per unit surface area. Emits fewer photons per unit time per unit surface area. Emits the same number of photons per unit time per unit surface area. Must necessarily have a higher luminosity. A and D 2. An ideal blackbody
A.
B.
C.
D.
Emits all wavelengths of radiation. Absorbs all wavelengths of radiation. Emits radiation based on temperature. All of the above. 3. An astronomer is observing a star of magnitude 18, but realizes that s/he needs 10
times as many photons to complete her measurement. S/he decides to look at a new
star. What is the faintest star s/he could observe and still get enough photons?
A. 15 B. 15.5 C. 16 D. 16.5 4. The energy of the nth level of the Hydrogen atom is proportional to
A.
B.
C.
D.
n2 1/n 1/n2 n 5. You take a spectrum of a continuum source, and see dark lines in the spectrum. What
happened to the photons at those wavelengths?
A. They were absorbed permanently by a medium. B. They were re-­‐emitted after some length of time in a different direction after being absorbed by a medium. C. They were never emitted by the source in the first place. D. Either B or C, depending on what kind of continuum emission it is. 6. Blackbody Alice peaks at 1000 Å. Blackbody Bob peaks at 9000 Å. Which of the
following are true? A.
B.
C.
D.
E.
Alice has a higher temperature than Bob. Bob has a higher intensity than Alice at 9000 Å. At every frequency, Alice has a higher intensity than Bob. A and B A and C 7. Whether we seen an emission spectrum or absorption spectrum depends on:
A.
B.
C.
D.
The thickness of the gas/plasma the radiation passes through.
The temperature of the gas.
The density of the gas.
All of the above.
Compare the following two telescopes:
Hale Telescope Keck 1 Diameter of mirror: 5.1 meters 10 meters Focal length: 16.9 meters 17.5 meters 8. Which telescope has a greater light-­‐gathering power (i.e., can collect more photons
per second)?
A. The Hale B. Keck 1 C. Both are equal 9. You decide to pull out your handy eyepiece, which as a 10mm focal length. Since no one
is around to stop you, you decide to look at the stars through each of these expensive telescopes. Which one will give you greater magnification? A. The Hale B. Keck 1 C. Both are equal 10. Synchrotron radiation is due to:
A.
B.
C.
D.
E.
Thermal emission from really hot plasma.
Electrons moving in a magnetic field with relativistic speeds.
Electrons being deflected by positive ions.
Very high order energy transitions in atoms.
Both A and C
11. Spectroscopic observations of some astronomical source can be used to: A.
B.
C.
D.
E.
F.
G.
H.
Measure its temperature. Measure its luminosity. Measure its chemical abundances. Measure its velocity. All four (ABCD) are correct All three of B, C, and D are correct All three of A, C, and D are correct None are correct Computational questions: 12. (4 points) The luminosity of the Sun is 4 x 1033 erg/s, and its radius is 7 x 1010 cm. a) What is the Sun’s effective temperature? b) What is the flux of sunlight on the Earth? c) You are tasked with building a solar power plant in the Arizona desert, using solar panels with 10% efficiency. How large an area must your solar panels cover to match the power output of a large nuclear power plant (about a GigaWatt)? Show your calculations (intermediate steps) here: Write your answers here: a) b) c) 13. (3 points) How far away could you read (“resolve the letters of”) a book with 3mm square type, using the Keck telescope on Mauna Kea, with typical “seeing” of half an arcsecond? What about with a Keck analogue in space? Since your vision is sensitive only to the optical regime, you must use an optical filter for your observations. Show your calculations (intermediate steps) here: Write your answers here: