Outline for 03 October Tuesday

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Group Question. . . How could you measure these curves experimentally?. Something is Fishy. In lecture and in the book, no explanation why it glows white before blue. . Can this figure help us explain?. . Can this figure help us explain?. Somewhatflat here. Near this temperature, this special combination of intensities is what we call white. Also, the realcurve is a little flatter near the peak.

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Outline for 03 October Tuesday

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1. Outline for 03 October (Tuesday) Questions about blackbody curves (20 minutes) Optics and Telescopes (Chapter 6 of text) (55 minutes)

2. Group Question

3. Something is Fishy In lecture and in the book, no explanation why it glows white before blue

4. Can this figure help us explain?

5. Can this figure help us explain?

6. Therefore The name you give a color depends on a somewhat complicated combination of wavelength intensities. What we call white is not a perfect mixture of photons of the visible spectrum.

9. So, what color is the sun in space?

10. So, what color is the sun in space?

11. So, what color is the sun in space? http://casa.colorado.edu/~ajsh/colour/Tspectrum.html

12. Online Quiz 7. If an object is a perfect blackbody then it emits no energy. it emits energy only at certain well-defined wavelengths called spectral lines. it emits energy with a continuous distribution that peaks at a certain wavelength dependent upon temperature.

13. Online Quiz 7. If an object is a perfect blackbody then it emits no energy. it emits energy only at certain well-defined wavelengths called spectral lines. it emits energy with a continuous distribution that peaks at a certain wavelength dependent upon temperature.

14. Online Quiz A perfect blackbody is so-called by scientists because it absorbs all energy falling upon it and emits a characteristic spectrum of radiation whose intensity as a function of wavelength depends only on its temperature. it absorbs all energy falling upon it and emits no energy at all, hence its name. the shape of the spectrum of energy emitted by it has a fixed shape independent of temperature and only the emitted intensity at each wavelength changes with the black-body's temperature.

15. Online Quiz A perfect blackbody is so-called by scientists because it absorbs all energy falling upon it and emits a characteristic spectrum of radiation whose intensity as a function of wavelength depends only on its temperature. it absorbs all energy falling upon it and emits no energy at all, hence its name. the shape of the spectrum of energy emitted by it has a fixed shape independent of temperature and only the emitted intensity at each wavelength changes with the black-body's temperature.

16. Online Quiz 17.In a beam of radiation from a blackbody, the amounts of energy per second at an ultraviolet wavelength of 300 nm and at an infrared wavelength of 800 nm are found to be equal. In this beam, how do the numbers of photons per second at each of these wavelengths compare? There will be more UV photons than IR photons. There will be equal numbers of photons at each of these wavelengths. There will be more IR photons than UV photons.

17. Online Quiz 17.In a beam of radiation from a blackbody, the amounts of energy per second at an ultraviolet wavelength (UV) of 300 nm and at an infrared (IR) wavelength of 800 nm are found to be equal. In this beam, how do the numbers of photons per second at each of these wavelengths compare? There will be more UV (300 nm) photons than IR photons. There will be equal numbers of photons at each of these wavelengths. There will be more IR (800 nm) photons than UV (300 nm) photons.

18. Outline for 03 October (Tuesday) Questions about blackbody curves Optics and Telescopes (Chapter 6 of text)

19. Key Words refraction/reflection converging/diverging lens focal point angular resolution magnification chromatic aberration

20. Key Questions Why are there so many telescopes in Hawaii? Why is our best most famous telescope orbiting Earth and not in Hawaii? What is the difference between optical and digital magnification (zoom)? How and when (but not why) does light (and other forms of electromagnetic radiation) bend? How does a telescope work? What is the difference between magnification and light-gathering power?

21. side note: What is the difference between optical and digital zoom?

22. side note: What is the difference between optical and digital zoom?

23. Practical note: What is the difference between optical and digital zoom?

24. You can create a digital zoom effect by taking a digital picture and expanding it (with photoshop, etc.) You can’t squeeze out more detail from the image (that is, increase the optical resolution), contrary to what you see on TV Therefore

25. Can explain lots about telescopes and other devices with only three optics principles

26. Principle 1 Light rays from distant object are nearly parallel

27. Principle 1 Light rays from distant object are nearly parallel

28. Principle 2 Light reflects off a flat mirror in the same way a basket ball would bounce on the floor (angle of incidence, i = angle of reflection, r)

29. Principle 3 prep

30. What happens, a, b, or c? As a beam of light passes from one transparent medium into another—say, from air into glass, or from glass back into air—the direction of the light can change This phenomenon, called refraction, is caused by the change in the speed of light

31. What happens, a, b, or c? As a beam of light passes from one transparent medium into another—say, from air into glass, or from glass back into air—the direction of the light can change This phenomenon, called refraction, is caused by the change in the speed of light

32. (an aside) Something is Fishy Look around in textbooks on physics and astronomy. You will find this analogy (or one with marching soldiers). What you won’t find is a physical explanation of how the wheels behave like light. This is an analogy that is useful for remembering which way light bends when going from one material to a denser material. It does not explain why it is so important that axle does not bend or give an explanation that uses the wave or particle picture of light

33. Principle 3 Light changes direction when it moves from one media to another (refraction). Use wheel analogy to remember which direction

34. Principle 3a Light changes direction when it moves from one media to another (refraction). Use wheel analogy to remember which direction

35. Principle 3b Same principle applies when going in opposite direction

36. Principle 3c At interface light diffracts and reflects (you can see your reflection in a lake and someone in lake can see you)

37. What happens to each beam? A B C

38. What happens?

39. A lens

40. A lens

41. What happens?

44. The Lines Converge

46. But you said different colors bend different amount!?

47. How I remember red bends less

48. How my optometrist remembers

49. But you said different colors bend different amount!?

50. Your turn

51. Your turn

52. Your turn

53. Your turn

61. Magnification is ratio of how big object looks to naked eye (angular diameter) to how big it looks through telescope

62. Although telescopes magnify, their primary purpose is to gather light

63. How much more energy does a 1 cm radius circular collector absorb than a 4 cm radius collector? Same 2x 4x 16x Need more info

64. How much more energy does a 1 cm radius circular collector absorb than a 4 cm radius collector? Same 2x 4x 16x Need more info

65. Reflecting telescope Previously I described a refracting telescope. The principles of reflection can be used to build a telescope too.

67. Problem: head blocks light!

68. Solutions

69. Independent reading: Read Chapter 6 and be able to answer … What is a prism spectrograph? What does it measure and what principles does it use? What is a radio telescope? What does it measure and what principles does it use?

70. A reflecting telescope used at prime focus does not suffer from chromatic aberration because  A) regardless of color, all wavelengths of light are reflected by the same amount.  B) the aluminum coating on the mirror absorbs light from all wavelengths except the range of interest to the astronomer.  C) the lens is perfectly formed so all colors of light travel through the lens along the same path.  D) the light has passed through only one lens. Ans:  A Section:  6-2 A spherical mirror suffers from spherical aberration because   A) the starlight is distorted by turbulence in the Earth's atmosphere.  B) the mirror sags under its own weight, distorting the image.  C) different parts of the mirror focus the light at different distances from the mirror.  D) different colors are focused at different distances from the mirror. Ans:  C Section:  6-2  A radio telescope  is very similar to a refracting optical telescope. is very similar to a reflecting optical telescope. is completely different in design from any optical telescope. combines major features of both refracting and reflecting optical telescopes. Ans:  B Section:  6-6  The two ranges of electromagnetic radiation for which Earth's atmosphere is reasonably transparent are  A) UV and radio waves. C) X rays and visible radiation.  B) visible and far infrared radiation. D) visible and radio radiation. Ans:  D Section:  6-7

71. Review Questions For Topics Covered in Lecture

72. (other questions) At what distance from the objective lens in a refracting telescope is the image formed (i.e., where would the photographic film or electronic detector be placed)?  A) immediately behind the lens to collect the most light  B) its diameter  C) its focal length  D) twice its focal length Ans:  C Section:  6-1

73. (other questions) At what distance from the objective lens in a refracting telescope is the image formed (i.e., where would the photographic film or electronic detector be placed)?  A) immediately behind the lens to collect the most light  B) its diameter  C) its focal length  D) twice its focal length

74. 1. The most important reason for the construction of large ground-based telescopes is to produce magnified images of distant objects. produce sharper images than smaller telescopes can produce. collect more light from distant objects. 2. A flash of light is transmitted simultaneously through two parallel tubes of length 1 km, one evacuated, the other filled with water. Detectors sense the arrival times of the light flash at the ends of these tubes. What will be the relationship between arrival times of these light flashes? The flashes of light will arrive simultaneously at the ends of the tubes because light always travels at the same speed. The flash will arrive earlier through the water-filled tube. The flash will arrive earlier through the evacuated tube. 3. After passing from the vacuum of space through a piece of glass and then passing back into the vacuum, photons of light will be traveling slower than when they entered the glass because they will have been slowed down by their passage through the glass. faster than before they entered the glass, having been accelerated by their passage through the glass. at the same speed as when they entered. 4. Which way does a light ray bend when it strikes the flat surface of a block of glass obliquely (i.e., at an angle to the surface) and passes into the glass? Toward the perpendicular to the surface, making a greater angle to the surface. A light ray does not change direction when it passes into the surface of the glass because the surface is flat. Light rays change their directions only through curved glass surfaces. Away from the perpendicular to the surface, bending toward the surface, making a smaller angle to the surface. 8. Many fortunate amateur astronomers have telescopes with primary mirrors 20 cm in diameter. The recently built Keck telescopes on Hawaii have mirrors 10 m in diameter. How much more light is collected by one of the Keck telescopes, compared to the amateur's telescope? 2,500 times greater About 7 times greater 50 times greater 16. In the primary mirror of a reflecting telescope, light of different wavelengths—such as red and blue light from a star—are focused with the red focus closer to the mirror than the blue focus. with the blue focus closer to the mirror than the red focus. at the same point. 19. What is diffraction of light? The spreading out of light waves after they pass through an opening such as the outer diameter of a lens or mirror. The bending of the path of a ray of light as it passes from one transparent medium to another; for example, from air to glass. The distortion in the image when light passes through a lens or reflects from a mirror, due to imperfections in the lens or mirror surface. 23. A factor that has become much worse for many observatories, and now severely limits the number of useful sites for astronomy in the world, is light pollution due to the increasing size of nearby cities. the number of satellites in orbit, which disturb observations when they pass in front of the object being observed. the weather, which has deteriorated significantly due to global warming.

75. (other questions) At what distance from the objective lens in a refracting telescope is the image formed (i.e., where would the photographic film or electronic detector be placed)?  A) immediately behind the lens to collect the most light  B) its diameter  C) its focal length  D) twice its focal length Ans:  C Section:  6-1 To correct for chromatic aberration in a refracting telescope a corrective lens is mounted next to the objective lens. In this corrective lens A) red light bends more than blue light. B) blue light bends more than red light. C) all colors bend the same amount. D) no bending is experienced by any of the colors. Ans:  A Section:  6-1 and Figure 6-7 In the reflection of a light beam from a flat mirror, the angle between the incident and reflected beams relative to the perpendicular to the surface of the mirror is  A) equal to the angle between the incident beam and the perpendicular.  B) equal to ½ the angle between the incident beam and the perpendicular.  C) always a right angle, or 90°.  D) twice the angle between incident beam and the perpendicular. Ans:  D Section:  6-2 

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