1. Lecture 13 ASTR 111 – Section 002

2. Optics and Telescopes

3. Reading

4. 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?

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

6. T Same amount of information if I just expand the original

7. Practical note: What is the difference between optical and digital zoom? T Much more information (detail)

8. 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

9. How much larger is a raw image of 800x800 pixels than one with 1600x1600 pixels?

10. 1600x1600 = 2,560,000 versus 800x800 = 640,000 4x 1600 800

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

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

14. Principle 1 Light rays from distant object are nearly parallel Collector

15. 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)

16. Principle 3 prep

17. 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 Axle and wheel from toy car or wagon Sidewalk Grass

18. 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 Axle and wheel from toy car or wagon Sidewalk Grass

19. Principle 3 Light changes direction when it moves from one media to another (refraction). Use wheel analogy to remember which direction normal 90 o Low index (e.g., air) Higher index (e.g. water)

20. Principle 3a Light changes direction when it moves from one media to another (refraction). Use wheel analogy to remember which direction normal 90 o Low index (e.g., air) Higher index (e.g. water)

21. Principle 3b Same principle applies when going in opposite direction normal 90 o Low index (e.g., air) Higher index (e.g. water)

22. Which normal does it bend towards/away from? Draw un-bent path and then bend a little towards or away from the nearest normal line.

23. Principle 3c At interface light diffracts and reflects (you can see your reflection in a lake and someone in lake can see you) Low index (e.g., air) Higher index (e.g. water) These angles are equal i r

24. What happens to each beam? ABCABC ABCABC ABCABC

25. What happens? ? ? ? zoom box

26. zoom box contents nearly flat when zoomed in zoom box contents

27. normal 90 o zoom box contents To figure out path, draw normal and un- bent path.

28. zoom box contents nearly flat when zoomed in normal 90 o zoom box contents Bends toward the normal.

29. What happens? ? ? ? zoom box

30. zoom box contents

31. 90 o

32. zoom box contents 90 o

34. The Lines Converge Input parallel lines converge to focal point F

35. F What happens to the beams here?

36. But you said different colors bend different amounts!?

37. This is chromatic aberration

38. How I remember red bends less

39. How my optometrist remembers Red light bends only a little Red light has little energy (compared to blue)

40. A B C

41. A normal Line A bends toward the normal Dashed line is path of line A if it does not bend When a light ray goes from air into glass (or water) it bends toward the normal

42. A normal Line A bends away from the normal Dashed line is path of line A if it does not bend When a light ray goes from air into glass (or water) it bends toward the normal. When a light ray goes from glass (or water) into air, it bends away from the normal.

43. A normal Line A bends away from the normal Dashed line is path of line A if it does not bend When a light ray goes from glass (or water) into air, it bends away the normal

44. What happens? ?

45. ?

46. Now we can explain

47. … rainbow color ordering

48. Sunlight diffraction reflection Water droplet Sunlight Finish drawing. Red light bends less on refraction. All light reflects at same angle

49. Now we can explain

50. … how an eye works

51. “Seeing” your optic nerve http://www.tedmontgomery.com/the_eye/optcnrve.html

52. … how an eye works Retina Info from distant object is concentrated on small area on retina Eye lens

53. … how an eye works Retina Eye lens Light from Sun Light from a distant lighthouse Sunlight lower than lighthouse light

54. … how an eye works Retina Eye lens Light from Sun Light from a distant lighthouse Sun appears lower than lighthouse light

55. Now we can explain

56. … how telescopes work

57. Magnification is ratio of how big object looks to naked eye (angular diameter) to how big it looks through telescope Telescope principles ½ o 10 o Magnification is 10/0.5 = 20x

58. Although telescopes magnify, their primary purpose is to gather light Telescope principles Collector

59. 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 Collector

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

63. Problem: head blocks light!

64. Solutions