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Physics 102 Final Exam
6 May 2002 Prof L. Weinstein

There are 65 questions. Please give a short explanation for all of your non-numerical answers. Show your work for all numerical answers. A table of physical data and a periodic table are at the back of the test.

1. (No explanation needed) Light will always travel from one place to another along a path of least
   1. distance
   2. time
   3. effort
   4. expense
   5. complication

2. The shortest plane mirror in which you can see your entire image is (hint: draw a picture for your explanation)
   1. half your height
   2. about 1/3 your height
   3. about 3/4 your height
   4. equal to your height
   5. dependent on the distance between you and the mirror
   
   
   
   
   
   
   

3. If you walk toward a mirror with a speed of 3 m/s, at what speed are you and your image approaching each other?

   
   
   
   
   
   
   

4. Some surfaces that are considered rough for reflecting infrared waves may be considered polished for reflecting
   1. radio waves
   2. light waves
   3. both
   4. neither
   5. need more information
   
   
   
   
   
   
   

5. (no explanation needed) Light refracts when traveling from air into glass because light
   1. has a different intensity in glass than air
   2. has a different frequency in glass than air
   3. has a different velocity in glass than air
   4. other

6. A beam of light emerges from water into air (see figure). Which light ray comes closest to the direction of the light ray in air?

   

   1. A
   2. B
   3. C
   4. D
   5. none come close

7. (no explanation needed) Rainbows are not usually seen as complete circles because
   1. the ground is usually in the way
   2. they are actually elliptical
   3. they have no bottom part
   4. raindrops are not perfectly round
   5. rainbows are actually arch shaped
   
   
   
   
   
   
   

8. If you wish to spear a fish with a regular spear, you should compensate for refraction between the air and the water and throw your spear (draw a picture for your explanation)
   1. directly at the sighted fish
   2. above the sighted fish
   3. below the sighted fish
   4. need more information
   
   
   
   
   
   
   

9. When a fish looks upward at angle of 45 degrees, does it see the sky or only a reflection of the bottom? Explain.

   
   
   
   
   
   
   

10. A planar wave of blue light passes through a very small hole. It then makes a bright spot on a screen a few meters away. If instead red light passes through the hole, the bright spot will be
    1. larger
    2. the same size
    3. smaller
    4. need more information
    
    
    
    
    
    
    

11. The colors seen when gasoline or oil forms a thin film on a puddle of water are a demonstration of
    1. refraction
    2. reflection
    3. diffraction
    4. polarization
    5. interference
    
    
    
    
    
    
    

12. What fraction of sunlight passes through a) an ideal polarizing filter?
    
    
    

    b) two ideal polarizing filters parallel to each other?
    
    
    

    c) two ideal polarizing filters at right angles to each other?
    
    
    

13. An inventor proposes to equip an office with a polarized source of background music and let those who prefer not to hear it wear polarizing earplugs. His idea is
    1. practical - it gives people a choice
    2. too expensive
    3. too much bother
    4. nonsense - you can't polarize a sound wave
    
    
    
    
    
    
    

14. Camera lenses coated with a layer of transparent material of the right thickness are almost completely non-reflective for light near the middle of the visible spectrum. They do, however, reflect a significant amount of
    1. green
    2. red
    3. white
    4. violet
    5. red and violet
    
    
    
    
    
    
    

15. Red laser light passes through two thin parallel slits and forms a diffraction pattern (a pattern of alternating dark and light spots) on a screen a few meters away. The distance between adjacent light spots is $d$. If the two slits are moved closer together, then the spacing between adjacent light spots will
    1. increase
    2. decrease
    3. remain the same
    4. need more information
    
    
    
    
    
    
    

16. You look at a thin yellow light source using diffraction glasses. You will see
    1. thin lines that are the same color as the yellow light
    2. thin lines of different colors
    3. either one, it depends on the light source
    4. need more information
    
    
    
    
    
    
    

17. (no explanation needed) To say that energy levels in an atoms are discrete, is to say that the energy levels are well defined and
    1. separate from one another
    2. separated from one another by the same energy increments
    3. continuous
    4. private

18. Light is emitted when an electron
    1. is boosted to a higher energy level
    2. makes a transition to a lower energy level
    3. swings around the atom in the same orbit
    4. other
    
    
    
    
    
    
    

19. (no explanation needed) A photon of which of the colors below has the longest wavelength?
    1. red
    2. orange
    3. yellow
    4. green
    5. blue

20. The frequencies of light emitted by an incandescent lamp (eg: a typical 100 W bulb) depends on the
    1. amount of electrical energy transformed
    2. temperature of the filament
    3. voltage applied to the lamp
    4. electrical resistance of the lamp
    5. energy differences between electron orbits in the gas inside the lightbulb
    
    
    
    
    
    
    

21. A paint pigment that emits red light when it is illuminated with blue light
    1. is fluorescent
    2. is phosphorescent
    3. is used in lasers
    4. is polarized
    5. doesn't exist
    
    
    
    
    
    
    

22. A star has a yellow spectral line at a frequency of $5.09 * 10^{14}$ Hz. If the star is moving toward us, we will observe the same spectral line at
    1. a slightly higher frequency
    2. a slightly lower frequency
    3. exactly the same frequency
    4. need more information
    
    
    
    
    
    
    

23. What is the energy of a red photon of frequency $4.6 * 10^{14}$ Hz? Give your answer in Joules and then convert it to electron-Volts.

    
    
    
    
    
    
    

24. What is the frequency of a blue photon of wavelength 440 nm (nano-meters)?

    
    
    
    
    
    
    

25. Light behaves as both a wave and a particle. Describe an experiment or a measurement that shows that light behaves as a particle.

    
    
    
    
    
    
    
    
    

26. When a clean surface of potassium metal is exposed to blue light, electrons are emitted. If the intensity of the blue light is increased, which of the following will also increase (choose all that apply)?
    1. the number of electrons ejected per second
    2. the maximum kinetic energy of the ejected electrons
    3. the threshold frequency of the ejected electrons
    4. the time lag between the absorption of blue light and the start of emission of the electrons
    5. none of these
    
    
    
    
    
    
    

27. A proton moving at 1700 m/s has a certain wavelength. If you double the speed, the wavelength will
    1. quarter
    2. halve
    3. stay the same
    4. double
    5. quadruple
    6. none of these
    
    
    
    
    
    
    

28. I want to use an electron beam to study the nucleus of the atom (diameter about $10^{-15}$ m). What electron momentum do I need?

    
    
    
    
    
    
    

29. Atoms can absorb energy from light or from heat. The absorbed energy lifts the electron wave from a low orbit near the nucleus to a higher orbit. When the atom radiates away the absorbed energy, the electron wave drops back down to the lower and smaller orbit. In the smallest orbit, the ground floor orbit, the electron cannot radiate any energy because
    1. it has zero kinetic energy
    2. the wave will not fit in a lower smaller orbit
    3. both
    4. neither
    
    
    
    
    
    
    

30. (no explanation needed) The atomic number of an element is always the same as the number of its
    1. electrons
    2. protons
    3. neutrons
    4. nucleons
    5. none of these

31. (no explanation needed) Different isotopes of an element have a different number of
    1. electrons
    2. protons
    3. neutrons
    4. none of these

32. The half-life of tritium, a radioactive isotope of hydrogen, depends on (circle all that apply)
    1. the number of atoms in the substance
    2. whether it is in the form of hydrogen molecules (H$_2$) or water molecules (H$_2$O)
    3. the temperature of the substance
    4. the age of the substance
    5. none of these
    
    
    
    
    
    
    

33. What are the daughter nuclei of the following decays: a) Radioactive potassium, ${}^{40}_{19}$K, decays by $\gamma$ emission.
    
    
    b) Sodium, ${}^{22}_{11}$Na, decays by positive $\beta$ emission.
    
    
    c) Americium, ${}^{242}_{95}$Am decays by $\alpha$ emission.

    
    
    
    

34. (no explanation needed) Most of the radioactivity we personally encounter comes from
    1. fallout from nuclear weapons tests
    2. nuclear power plants
    3. medical x-rays
    4. other man-made sources
    5. the natural environment

35. The half-life of Carbon 14 is 5730 years. If a 1-gram sample of old carbon is 1/8 as radioactive as 1 gram of a current sample, how old is the sample?

    
    
    
    
    
    
    

36. In the fissioning of Uranium, a chain reaction is caused by
    1. the enormous energy release
    2. the kinetic energy of the decay products
    3. ejected neutrons
    4. the conversion of mass to energy
    5. none of these
    
    
    
    
    
    
    

37. Fissioning Helium would
    1. release energy
    2. require energy
    3. neither
    4. need more information
    
    
    
    
    
    
    

38. Two glasses are filled with water to the same depth. One glass has straight vertical sides and the other is V shaped (see picture). Which glass has the higher pressure at the bottom?

    

    1. straight sides
    2. V shaped
    3. both the same
    4. need more information

39. At sea level, even a perfect vacuum can raise water only 10 meters up a straw. How high can you raise water up a straw in Bozeman, Montana (elevation 5000 feet)?
    1. less than 10 m
    2. 10 m
    3. more than 10 m
    4. need more information
    
    
    
    
    
    
    

40. Salt water is slightly denser than fresh water. Will a 1 cubic meter, 3.5 ton rock feel a greater buoyant force at a depth of 100 m in the ocean or in a freshwater lake (assuming the water is the same temperature)?
    1. the ocean
    2. the lake
    3. both the same
    4. need more information
    
    
    
    
    
    
    

41. Salt water is slightly denser than fresh water. Will a 3.5 ton boat feel a greater buoyant force floating in the ocean or in a freshwater lakee (assuming the water is the same temperature)?
    1. the ocean
    2. the lake
    3. both the same
    4. need more information
    
    
    
    
    
    
    

42. A submarine in the arctic ocean (where the water is $4^o$ C from surface to bottom) experiences a buoyant force of 23,000 tons when it is at a depth of 1000 m. What is the buoyant force on the submarine when it is at a depth of 100 m?
    1. less than 23,000 tons
    2. approximately 23,000 tons
    3. more than 23,000 tons
    4. need more information
    
    
    
    
    
    
    

43. Blood flows through arteries to all parts of our body. An aneurysm is a widening of an artery (see figure, the arrow shows the direction of blood flow). How does the blood pressure in the aneurysm compare to the blood pressure in the rest of the artery?

    

    1. the aneurysm blood pressure is higher $\rightarrow$ the aneurysm will get bigger (uh-oh!)
    2. the aneurysm blood pressure is lower $\rightarrow$ the aneurysm will get smaller (whew!)
    3. the pressures are the same
    4. need more information
    
    
    
    
    
    
    

44. A bag is filled with 2 cubic meters of an ideal gas at a pressure of 1 atmosphere and a temperature of $20^o$ C (about 68$^o$ F). If the temperature is doubled to $40^o$ C without changing the pressure, then the volume of the gas becomes
    1. less than 2 m$^3$
    2. more than 2 m$^3$ but less than 4 m$^3$
    3. 4 m$^3$
    4. more than 4 m$^3$
    5. need more information
    
    
    
    
    
    
    

45. How and why would placing a large aluminum nail through a potato before baking it affect the cooking time?

    
    
    
    
    
    
    

46. You add 100 J of heat each to 100 g of water and to 100 g of iron. Which material has the greatest increase in internal energy?
    1. the water
    2. the iron
    3. both the same
    4. need more information
    
    
    
    
    
    
    

47. You add 100 J of heat each to 100 g of water and to 100 g of iron. Which material has the greatest increase in temperature?
    1. the water
    2. the iron
    3. both the same
    4. need more information
    
    
    
    
    
    
    

48. Which gets hotter in sunlight, a black car or a white car?
    1. black
    2. white
    3. both the same
    4. need more information
    
    
    
    
    
    
    

49. Which gets colder on a clear night, a black car or a white car?
    1. black
    2. white
    3. both the same
    4. need more information
    
    
    
    
    
    
    

50. A mechanic is testing an air conditioner by running it on a workbench in an isolated room. What happens to the temperature of the room?
    1. it increases
    2. it decreases
    3. it stays the same
    4. need more information
    
    
    
    
    
    
    

51. What input energy is required if an engine performs 200 J of work and exhausts 400 J of heat to the cold reservoir?

    
    
    
    
    
    
    

52. What is the efficiency of an ideal heat engine with a boiler at 200$^o$ C that exhausts its heat at room temperature (27$^o$ C)?

    
    
    
    
    
    
    

53. We can treat people's legs as pendulums to analyze how they walk. Which legs take longer to swing back and forth, those of tall people or short people?
    1. tall
    2. short
    3. both the same
    4. need more information
    
    
    
    
    
    
    

54. You stand just to the side of the low point of a child's swing and always push the child in the same direction. The period of the swing is 3 s. You push the swing at even intervals. Which intervals will make the swing go very high (resonance)? Circle all that apply.
    1. pushing once every 1 s
    2. pushing once every 2 s
    3. pushing once every 3 s
    4. pushing once every 4 s
    5. pushing once every 5 s
    6. pushing once every 6 s
    7. none of the above
    
    
    
    
    
    
    

55. If you double the speed of a wave while keeping the wavelength fixed, what happens to the period?
    1. it gets 4 times smaller
    2. it gets 2 times smaller
    3. it stays the same
    4. it gets 2 times bigger
    5. it gets 4 times bigger
    6. need more information
    
    
    
    
    
    
    

56. When you yell at a friend, are the air molecules that strike his ear the same ones that left your lungs?
    1. Yes
    2. No
    
    
    
    
    
    
    

57. (no explanation needed) When you see your friend, are the photons that reach your eyes the same ones that bounced off her?
    1. Yes
    2. No

58. Draw diagrams to show the standing wave pattern for the first three harmonics of a rope fixed at both ends.

    
    
    
    
    
    
    
    
    

59. A pipe organ has pipes of many different lengths. Which ones make the highest notes?
    1. the shortest ones
    2. the middle ones
    3. the longest ones
    4. need more information
    
    
    
    
    
    
    

60. Your real estate agent assures you that extra traffic from the new shopping mall will only increase noise from cars by about 15%, from 60 dB to 70 dB. What does this imply about the number of cars using the street?
    1. It will increase by less than 15%
    2. It will increase by about 15%
    3. It will increase by more than 15%
    4. need more information
    
    
    
    
    
    
    

61. A green cloth is illuminated by magenta light. What color does it appear?

    
    
    
    
    
    
    

62. If you mix cyan and magenta paint, what color paint will result? (ie: if you shine white light on the mixture, what color will it appear?)

    
    
    
    
    
    
    

63. Light travels through the air at $3\cdot 10^8$ m/s and hits a piece of glass. The light in the glass travels more slowly than the light in the air (at about $2\cdot 10^8$ m/s). When the light re-emerges into the air its speed is
    1. less than $2\cdot 10^8$ m/s
    2. $2\cdot 10^8$ m/s
    3. between 2 and $3\cdot 10^8$ m/s
    4. $3\cdot 10^8$ m/s
    5. more than $3\cdot 10^8$ m/s
    6. need more information
    
    
    
    
    
    
    

64. Why don't we see colors at night when there is a half-moon?

    
    
    
    
    
    
    
    
    

65. Rank the following photons from least to most energetic: radio, ultraviolet, blue, infrared, gamma ray, green