Lecture 12

2. Lecture 12 ASTR 111 – Section 002

3. Outline • Quiz Discussion • Finish a few slides from last lecture • Light (Reading is Chapter 5)

4. Quiz Discussion • 75% Computing your grade – will not cover in class • 66% Photons through a hole – will cover in class • Can we finish going over lecture 12 in class? • Are you going to post the answers to lecture 12? • When is the next exam scheduled? • What acts as a nature's prism to create a rainbow in the sky?   • If enacted, will clickers be mandatory?   • I think we should use iclickers. Wouldn't it be easier than texting? • How many pets do you really have? • Can you review fully before the next exam? • Does it bother you when people come 45 minutes late to lecture and slam there stuff around and make a lot of noise? Because it really bothers me.   • Why is this class getting exponentially more difficult?

5. Outline • Quiz Discussion • Finish a few slides from last lecture • Light (Reading is Chapter 5)

6. Measurements in Astronomy • In astronomy, we need to make remote and indirect measurements • Think of an example of a remote and indirect measurement from everyday life

7. Using Light • Light has many properties that we can use to learn about what happens far away • Light interacts with matter in a special way

8. Only photons with special wavelengths will interact with atom How will this affect what a person will see at point X? When is the atom “hotter”? X From Universe 7e Section 5.2 online material http://bcs.whfreeman.com/universe7e/pages/bcs-main.asp?v=chapter&s=05000&n=00020&i=05020.02

9. Why is UV light usually blamed for skin cancer? What is special about it compared to other light sources?

10. DNA “absorbs” or “is excited by” UVB radiation. • This causes a chemical reaction to take place that modifies DNA. • Why doesn’t UVA affect DNA like this? http://earthobservatory.nasa.gov/Features/UVB/Images/dna_mutation.gif

12. Cloud of Gas A prism bends photons more or less depending on their wavelength

13. Cloud of Gas A prism bends photons more or less depending on their wavelength

14. What will the spectrum look like here?

15. Emission line spectrum

16. A blackbody emits photons with many energies (wavelengths) – a continuous spectrum Continuous Spectrum

17. What will the spectrum look like here?

18. Absorption Spectrum

19. Three types of spcetra

20. What type of spectrum is produced when the light emitted from a hot, dense object passes through a prism? • What type of spectrum is produced when the light emitted directly from a cloud of gas passes through a prism? • Describe the source of light and the path the light must take to produce an absorption spectrum • There are dark lines in the absorption spectrum that represent missing light. What happened to this light that is missing in the absorption line spectrum? From Lecture Tutorials for Introductory Astronomy, page 61.

21. Each chemical element produces its own unique set of spectral lines

23. Stars like our Sun have low-density, gaseous atmospheres surrounding their hot, dense cores. If you were looking at the spectra of light coming from the Sun (or any star), which of the three types of spectra would be observed? • If a star existed that was only a hot dense core and did not have a low-density atmosphere surrounding it, what type of spectrum would you expect this particular star to give off? • Two students are looking at a brightly lit full Moon, illuminated by reflected light from the Sun. Consider the following discussion between two students about what the spectrum of moonlight would look like: • I think moonlight is just reflected sunlight, so we will see the Sun’s absorption line spectrum. • I disagree, an absorption spectrum has to come from a hot, dense object. Since thie Moon is not a hot, dense object, it can’t give off an absorption line spectrum. Do you agree or disagree with either or both of these students? Explain your reasoning. c c c

24. Stars like our Sun have low-density, gaseous atmospheres surrounding their hot, dense cores. If you were looking at the spectra of light coming from the Sun (or any star), which of the three types of spectra would be observed? • If a star existed that was only a hot dense core and did not have a low-density atmosphere surrounding it, what type of spectrum would you expect this particular star to give off? • Two students are looking at a brightly lit full Moon, illuminated by reflected light from the Sun. Consider the following discussion between two students about what the spectrum of moonlight would look like: • I think moonlight is just reflected sunlight, so we will see the Sun’s absorption line spectrum. • I disagree, an absorption spectrum has to come from a hot, dense object. Since thie Moon is not a hot, dense object, it can’t give off an absorption line spectrum. Do you agree or disagree with either or both of these students? Explain your reasoning. c c c

25. Stars like our Sun have low-density, gaseous atmospheres surrounding their hot, dense cores. If you were looking at the spectra of light coming from the Sun (or any star), which of the three types of spectra would be observed? • If a star existed that was only a hot dense core and did not have a low-density atmosphere surrounding it, what type of spectrum would you expect this particular star to give off? • Two students are looking at a brightly lit full Moon, illuminated by reflected light from the Sun. Consider the following discussion between two students about what the spectrum of moonlight would look like: • I think moonlight is just reflected sunlight, so we will see the Sun’s absorption line spectrum. • I disagree, an absorption spectrum has to come from a hot, dense object. Since thie Moon is not a hot, dense object, it can’t give off an absorption line spectrum. Do you agree or disagree with either or both of these students? Explain your reasoning. c c c

26. Stars like our Sun have low-density, gaseous atmospheres surrounding their hot, dense cores. If you were looking at the spectra of light coming from the Sun (or any star), which of the three types of spectra would be observed? • If a star existed that was only a hot dense core and did not have a low-density atmosphere surrounding it, what type of spectrum would you expect this particular star to give off? • Two students are looking at a brightly lit full Moon, illuminated by reflected light from the Sun. Consider the following discussion between two students about what the spectrum of moonlight would look like: • I think moonlight is just reflected sunlight, so we will see the Sun’s absorption line spectrum. • I disagree, an absorption spectrum has to come from a hot, dense object. Since thie Moon is not a hot, dense object, it can’t give off an absorption line spectrum. Do you agree or disagree with either or both of these students? Explain your reasoning.

27. Imagine that your are looking at two different spectra of the Sun. Spectrum #1 is obtained using a telescope that is in a high orbit far above Earth’s atmosphere. Spectrum #2 is obtained using a telescope located on the surface of Earth. Label each spectrum below as either Spectrum #1 or Spectrum #2.

28. Imagine that your are looking at two different spectra of the Sun. Spectrum #1 is obtained using a telescope that is in a high orbit far above Earth’s atmosphere. Spectrum #2 is obtained using a telescope located on the surface of Earth. Label each spectrum below as either Spectrum #1 or Spectrum #2. Spectrum #2 (Near surface) Spectrum #1 (High above surface)

29. Would this make sense? This dark line was removed Spectrum #2 (Near surface) Spectrum #1 (High above surface)

30. Energy and electromagnetic radiation Planck’s law relates the energy of a photon to its frequency or wavelength E = energy of a photon h = Planck’s constant c = speed of light l = wavelength of light The value of the constant h in this equation, called Planck’s constant, has been shown in laboratory experiments to be h = 6.625 x 10–34 J s

31. Which electromagnetic wave has a higher energy: one with f=10 cycles per second or f=1 cycles per second?

32. Three Temperature Scales

33. Color and Temperature

34. An opaque object emits electromagnetic radiationaccording to its temperature

35. ! (An aside)

36. http://www.straightdope.com/mailbag/mhotflame.html Blue: Hot or Not?

38. If blue light has higher energy, and energy is proportional to temperature, why are my cold spots blue?

39. If it is not opaque (or a perfect blackbody), relationship between color that you see and temperature are more complicated.

40. Why do we associate blue with cold and red with hot? • Lips turn blue when cold • Ice takes on a blue-ish tint • Face turns red when hot • Red is the first thing you see when something is heated (usually don’t see much blue)

41. What you see depends on if it is a result of • Absorption (light reflected off your face or light reflected by a plant) • Emission (light from a flame or a heated bar)

42. !

44. Blackbody Definition • Does not reflect incoming radiation, only absorbs • Emits radiation, depending on temperature • Temperature and emitted radiation intensity follow a special relationship One way of creating a blackbody The “hole” is the blackbody. Photon enters If hole is very small, what is probability that it exits?

45. Wien’s law and the Stefan-Boltzmann law are useful tools for analyzing glowing objects like stars • A blackbody is a hypothetical object that is a perfect absorber of electromagnetic radiation at all wavelengths • Stars closely approximate the behavior of blackbodies, as do other hot, dense objects

46. Blackbodies do not always appear black! • The sun is close to being a “perfect” blackbody • Blackbodies appear black only if their temperature very low

47. Special Relationship For Intensity, think photons/second on a small area Intensity Wavelength

48. Question • Why is photon/second similar to energy/second? How are they related?

49. Watt? Energy Flux? Using

50. Flux Flux is a measure of how much “stuff” crosses a small patch in a given amount of time. Can have flux of green photons, red photons, etc.