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
This may be a good time to have them work in groups to come up with answers. You may also want to try to get them to think and debate the distinction between what defines a direct versus an indirect measurement.

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. Example with colored balls and a child who only likes certain colors
Example with colored balls and a child who only likes certain colors. He never stops to grab certain colors that roll by. When he catches certain colors, he holds on to them for a while before tossing them in a random direction.

9. X 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”?
From Universe 7e Section 5.2 online material X
From Universe 7e Section 5.2 online material

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

11. 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?

13. Cloud of
Gas
A cloud of gas will emit only certain frequencies. Better to bring colored balls and throw them to demonstrate. What will you see on the wall?
A prism bends photons more or less depending on their wavelength

14. Cloud of
Gas
A cloud of gas will emit only certain frequencies. Better to bring colored balls and throw them to demonstrate. What will you see on the wall?
A prism bends photons more or less depending on their wavelength

15. What will the spectrum look like here?

16. Emission line spectrum

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

18. What will the spectrum look like here?
Note that the original image said “Cloud of cool gas”. I covered up the “cool” because some people thought it would mean that the gas was so cold that we would not see anything on the spectrum (they were actually thinking about the radiated power amplitude according to the blackbody curve being very low in the visible range for a cold gas).
What will the spectrum look like here?

19. Absorption Spectrum

20. Three types of spcetra

21. 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.

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

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. c c c

27. 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.

28. Imagine that your are looking at two different spectra of the Sun
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.

29. Spectrum #2 (Near surface) Spectrum #1 (High above surface)
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)

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

31. 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 = x 10–34 J s

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

33. Three Temperature Scales

34. Color and Temperature

35. An opaque object emits electromagnetic radiation according to its temperature

36. !
(An aside)

37. Blue: Hot or Not?

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

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

41. 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)

42. 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)

43. What you see depends on if it is a result of
Absorption and reflection (light reflected off your face or light reflected by a plant)
Scattering (Absorption and then emission at a random angle as in a gas)
Emission (light from a flame or a heated bar)
Blackbody has simple absorption and emission spectra
Elements have simple absorption and emission spectra
Other objects have complex absorption and emission spectra, and to explain how the color relates to the temperature, you usually need to combine Absorption, Emission, and Scattering effects.
General trend is emitting object get bluer when heated and absorbing objects get bluer when cooled.

44. !

46. 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?

47. 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
Eventually photon excites an atom or molecule in the wall.

48. 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
Excited molecule emits photons at different wavelengths (according to its spectra). Does not need to re-emit the same wavelength!

49. 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

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

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

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

53. Watt? Energy Flux?
Using

54. 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.

55. Blackbodies and Astronomy

56. Blackbody Laws
Stefan-Boltzmann Law – relates energy output of a blackbody to its temperature
Wein’s law – relates peak wavelength output by a blackbody to its temperature

57. Special Relationship
For Intensity, think photons/second on a small area
Energy Flux Intensity
Wavelength

58. Stefan-Boltzmann Law
A blackbody radiates electromagnetic waves with a total energy flux F directly proportional to the fourth power of the Kelvin temperature T of the object:

59. Special Relationship
Stefan-Boltzmann Law tells us that if we add up the energy from all wavelengths, then the total energy Flux
Energy Flux Intensity
Wavelength

60. Special Relationship
Wien’s law tells us that lmax depends on temperature
Max intensity at lmax
Energy Flux Intensity
Wavelength
lmax

61. Special Relationship Sketch this curve for larger and smaller T
Energy Flux Intensity
Wavelength

63. Wavelength of peak decreases as temperature increases
At high wavelengths, intensity goes to zero
Overall amplitude increases with Temperature
As wavelength goes to zero, intensity goes to zero

64. Color and Temperature

65. What would this object look like at these three temperatures?

67. Why does it glow white before blue?

68. Can this figure help us explain?

69. Can this figure help us explain?
Near this temperature, this special combination of intensities is what we call white. Also, the real
curve is a little flatter near the peak
Can this figure help us explain?

70. The Sun does not emit radiation with intensities that exactly follow the blackbody curve

71. If “white” was actually defined by the ideal blackbody curve, perhaps we could add a little green to white …

72. So, what color is the sun in space?
Solid green
square

73. So, what color is the sun in space?
Add a little green to white background by making
solid green
square mostly transparent

74. If “white” was actually defined by the ideal blackbody curve, this would (sort of*) make sense …
* A stream of photons of wavelength that we call green would actually be perceived as a mixture of red, green, and blue by our eye, so calculation is more complicated …
But what we call white is actually not the ideal blackbody curve. See

75. So, what color is the sun in space?
Left side is white
Right side is (should be) a
little “pinker”

76. D65 is reference “white” It is determined using a detector that measures the Flux of photons

77. 5 A B
Figure 1 4 C 3
Energy Flux 2 1

78. Which curve represents an ideal blackbody?
Curve A
Curve B
Curve C

79. Which curve represents an ideal blackbody?
Curve A
Curve B
Curve C

80. If the object in Figure 1 were increased in temperature, what would happen to curves A, B, and C?

81. If the object in Figure 1 were increased in temperature, what would happen to curves A, B, and C?
All would increase in amplitude. Peak would shift to left.

82. Curve C is more jagged. The locations where the curve C is small correspond to
Spectral lines of a blackbody
Spectral lines of atmospheric molecules
Instrumentation error
Diffraction lines
Spectral lines of the lens used to the light into colors

83. Curve C is more jagged. The locations where the curve C is small correspond to
Spectral lines of a blackbody
Spectral lines of atmospheric molecules
Instrumentation error
Diffraction lines
Spectral lines of the lens used to the light into colors

85. What is the intensity of curve B at 550 nm?
Impossible to tell; 550 nm is not shown in this figure
Nearest 4
Nearest 3
Nearest 1
Nearest 0.5

87. What is the intensity of curve B at 550 nm?
Impossible to tell; 550 nm is not shown in this figure
Nearest 4
Nearest 3
Nearest 1
Nearest 0.5

88. The moon has no atmosphere
The moon has no atmosphere. If you measure the spectrum of the same object as that measured in Figure 1 from its surface instead of from Earth’s,
Curves B and C would not change
Curve C would look more like A
Curve C would look more like B
Curve B would look more like A
Curve B would look more like C

89. Venus has no atmosphere. If you measure the spectrum from its surface,
Curves B and C would not change
Curve C would look more like A
Curve C would look more like B
Curve B would look more like A
Curve B would look more like C

90. White light is composed of
Equal intensities of all colors of the rainbow
Unequal intensities of all colors of the rainbow
Equal number of photons of all colors of the rainbow
Unequal number of photons of all colors of the rainbow
Equal numbers of red, green, and blue photons

91. White light is composed of
Equal intensities of all colors of the rainbow
Unequal intensities of all colors of the rainbow
Equal number of photons of all colors of the rainbow
Unequal number of photons of all colors of the rainbow
Equal numbers of red, green, and blue photons

92. Does a blackbody have color?
Yes, and they all appear the color of the sun
No, you cannot see a blackbody
Yes, but its depends on its temperature
Maybe, it depends on if it is an ideal blackbody

93. Does a blackbody have color?
Yes, and they all appear the color of the sun
No, you cannot see a blackbody
Yes, but its depends on its temperature
Maybe, it depends on if it is an ideal blackbody

94. Why is the best reason for putting a telescope in orbit?
Closer to stars
Better view of celestial sphere
The speed of light is higher in space
Less atmospheric interference
Cost

95. Why is the best reason for putting a telescope in orbit?
Closer to stars
Better view of celestial sphere
The speed of light is higher in space
Less atmospheric interference
Cost

96. Bonus
How would blackbody curve change as you moved closer to the object?