PHYS 107 ASTRONOMY 4

• Question 1

  1 out of 1 points

  	Radio telescopes are generally much larger than optical telescopes:
  	Selected Answer:   to achieve the same angular resolution Correct Answer:  to achieve the same angular resolution Response Feedback: Correct, the resolution depends on wavelength.  The angular resolution is worse at long wavelengths, so to separate two objects on the sky you need a much larger radio telescope.

• Question 2

  1 out of 1 points

  	Both of the Keck telescopes on Mauna Kea have diameters of 10 meters compared to the 2.5 meter Hooker telescope on Mount Wilson which Hubble used to discover the expansion of the Universe.  How much larger is the light gathering power of one of the Keck telescopes compared to that of the Hooker telescope?
  	Selected Answer:   16x Correct Answer:  16x Response Feedback: Correct, since one of the Keck telescopes has 4 times the diameter of the Hooker telescope, its area is 16 times larger so it gathers 16 times more light.

• Question 3

  1 out of 1 points

  	From lowest energy to highest energy, which of the following correctly orders the different categories of electromagnetic radiation?
  	Selected Answer:    radio, infrared, visible light, ultraviolet, X-rays, gamma rays Correct Answer:    radio, infrared, visible light, ultraviolet, X-rays, gamma rays Response Feedback:  Correct as shown in Figure 5.2

• Question 4

  1 out of 1 points

  	If a material is transparent, then it:
  	Selected Answer:    transmits light well. Correct Answer:    transmits light well. Response Feedback:  Correct, transparent materials transmit light well.

• Question 5

  1 out of 1 points

  	Which of the following statements about electrical charge is true?
  	Selected Answer:    A positive charge and a negative charge will attract each other. Correct Answer:    A positive charge and a negative charge will attract each other. Response Feedback:  Correct opposite charges attract, like charges repel

• Question 6

  1 out of 1 points

  	Wien's law says that the peak of a black-body spectrum is at:
  	Selected Answer:   a larger (redder) wavelength for a cooler object Correct Answer:  a larger (redder) wavelength for a cooler object Response Feedback: Correct, this is the definition of Wien's law, the peak wavelength is inversely proportional to the temperature. Thus hotter objects has a bluer (more energetic) peak emission wavelength and cooler objects have a redder (less energetic) peak emission wavelength (see figure 2a on page 58 in lectures-tutorials or figure 5-2 on page 80 in Perspectives on Astronomy).

• Question 7

  1 out of 1 points

  	Incandescent light bulbs have a temperature of ~2700 K, this means that their light is emitted mostly:
  	Selected Answer: d.  at infrared wavelengths Correct Answer: b. at infrared wavelengths Response Feedback: Correct, this is why incandescent light bulbs are relatively inefficient, most of their light comes out a wavelengths longer than can be seen by humans.

• Question 8

  0 out of 1 points

  	We can see most other objects in the room right now because they:
  	Selected Answer:    emit visible light. Correct Answer:    reflect visible light Response Feedback:  Incorrect, we see optical light reflected by these objects, all of which are too cool to emit their own light (the exception of course being light bulbs).

• Question 9

  1 out of 1 points

  	Which of the following is a major benefit of the Hubble Space Telescope?
  	Selected Answer:   It has very high resolution since it is above the distorting effects of Earth's atmosphere. Correct Answer:  It has very high resolution since it is above the distorting effects of Earth's atmosphere. Response Feedback: Correct, motion in the Earth's atmosphere "blurs" images and results in poor resolution for ground-based telescopes (although this is being solved in part by adaptive optics, especially at wavelengths in the near infrared).

• Question 10

  1 out of 1 points

  	Which fills in the following statement correctly? The portion of the electromagnetic spectrum with the greatest energy has the (______) frequency and the (________) wavelength.
  	Selected Answer: b.  highest frequency and shortest wavelength Correct Answer: c. highest frequency and shortest wavelength Response Feedback: Correct, since wavelength and frequency are inverses of each other (high frequency = short wavelength, and low frequency = long (large) wavelength), and the energy is inversely proportional to the wavelength (so high energy = short wavelength).  Compare to your answer for #6 on page 46 in lectures-tutorials.

• Question 11

  0 out of 1 points

  	You have two objects of the same temperature but differing sizes, which one gives off more energy?
  	Selected Answer: d.  You can't be sure from the information given Correct Answer: c. The larger one Response Feedback: Incorrect, since the energy output L=constants*T4 * R2, and they are the same temperature the larger one gives off more energy.  Does this agree with your answers to 1B and 3 on page 53 in lectures-tutorials? You can also see how if the size is constant, the hotter object "wins" (see 1A and 2). This sort of relationship is different than the force of gravity or light flux, where we saw that these fell off as distance squared.  Note that larger size (greater R) means more surface area, but that more surface area means more area for light to come from instead of more area for light (or the force of gravity) to be diluted.

• Question 12

  1 out of 1 points

  	When an atom absorbs a high energy photon and an electron is ejected we call this
  	Selected Answer: e.  ionization Correct Answer: c. ionization Response Feedback: Correct, ionization is when a photon of enough energy is absorbed by an atom to eject an electron (and recombination is the reverse process and results in the emission of a photon).

• Question 13

  1 out of 1 points

  	Imagine two stars, one is 2xhotter, how much larger must the other be to be the same brightness
  	Selected Answer:   4x larger Correct Answer:  4x larger Response Feedback: Correct, the luminosity of an object goes as L=constant * T4 R2 so if the L's are identical if one star is 2 hotter than the other, the radius must be 4*larger.  Since 2*hotter means that L is 2*2*2*2 = 16*larger, and the square root of 16 is 4. This re-enforces 1D and 4 on page 53 of lectures-tutorials, as well as question #11 on page 55 in lectures-tutorials, where we argue that star X must be larger than star S to put out the same amount of light.

• Question 14

  1 out of 1 points

  	If you have two objects, one is smaller but higher temperature and one is larger but lower temperature which gives off more energy?
  	Selected Answer: d.  you can't be sure from the information given Correct Answer: d. you can't be sure from the information given Response Feedback: Correct, as the energy goes as L=constants * T4 * R2.  Since it depends on how much hotter the small object is compared to how much larger the big object is, and since this information is not given you can't tell (see #1 D and #4 on page 53 in lectures-tutorials, did you agree?). While for the same size the hotter gives out more energy (see #1 A and #2), and for the same temperature the larger size gives off more energy (see #1 B and #3), you can't tell for sure for for this case.  They even can give off exactly the same amount if the difference in size is large enough.

• Question 15

  1 out of 1 points

  	The Chandra X-ray satellite is in space because:
  	Selected Answer:   X-rays do not penetrate the Earth's atmosphere Correct Answer:  X-rays do not penetrate the Earth's atmosphere Response Feedback: Correct, short wavelengths such as gamma rays, X-rays and ultraviolet light do not penetrate the Earth's atmosphere.

• Question 16

  1 out of 1 points

  	Which of the following is the highest energy photon?
  	Selected Answer: b.  X-ray photon Correct Answer: b. X-ray photon Response Feedback: Correct, since the X-ray photon has the shortest wavelength of the four listed it has the highest energy since energy is inversely proportional to wavelength.

• Question 17

  1 out of 1 points

  	The wavelength of a wave is:
  	Selected Answer:    the distance between two adjacent peaks of the wave. Correct Answer:    the distance between two adjacent peaks of the wave. Response Feedback:  Correct, the wavelength measures the distance between adjacent peaks (or two adjacent troughs)

• Question 18

  1 out of 1 points

  	Which diagram shows the absorption of the lowest energy photon?
  	Selected Answer: b.  B Correct Answer: b. B Response Feedback: Correct, b and d  show absorption (electron moving from inner lower energy state to outer higher energy state) and the electron in b is moving  less so has lower energy.

• Question 19

  0 out of 1 points

  	Which star has the higher temperature?
  	Selected Answer: a.  Star A Correct Answer: c. both are equal Response Feedback: Incorrect, both stars have the same peak wavelength so both are the same temperature (8000 K).  This is an application of Wien's Law.

• Question 20

  1 out of 1 points

  	The SOFIA project is a 2.5 meter telescope designed to observe in the infrared built into a 747 airplane.  The main reason to fly such a plane is:
  	Selected Answer:   to travel above much of the absorption and emission of the Earth's atmosphere Correct Answer:  to travel above much of the absorption and emission of the Earth's atmosphere Response Feedback: Correct, the Earth emits the bulk of its light in the infrared and greenhouse gasses in our atmosphere block infrared light, getting above this emission and absorption lets astronomers see stars at long wavelengths.