brightnightgallery/

1. From a location with little light pollution (+ some camera tricks) http://www.brightnightgallery.com/

2. Lecture 8 ASTR 111 – Section 002

3. Mid-term evaluations/grades • Required for Freshman and Sophomores • Due by October 17th • I entered mid-term grades for everyone • Included everything before Quiz 6. • Used 90% Exam 1 and 10% Quizzes • If you missed Exam 1, used your Quiz ave.

4. Outline • Quiz Discussion • Quiz solutions shown in class are now posted online • Light • Suggested reading: Chapter 5.3-5.4 and 5.9 of textbook • Optics and Telescopes • Suggested reading: Chapter 6.1-6.4

5. Volume Increase r by a factor of 2 and Volume increases by 2x2x2 = 8 The radius changed from 0.5 to 1.0. Compute Volume using radius of 0.5 m and 1.0 m to see if you still get 8x the volume! 2 meters 1 meter Sphere Sphere

6. Area Increase r by a factor of 2 and Surface Area increases by 2x2 = 4 The radius changed from 0.5 to 1.0. Compute Area using radius of 0.5 m and 1.0 m to see if you still get 4x the Area! 2 meters 1 meter Disk Disk

7. Doppler Effect

8. Doppler animations • http://www.colorado.edu/physics/2000/applets/doppler2.html • http://www.grc.nasa.gov/WWW/K-12/airplane/sndwave.html

9. The wavelength of a spectral line is affected by therelative motion between the source and the observer

10. Doppler Shifts • Red Shift: The object is moving away from the observer • Blue Shift: The object is moving towards the observer Dl/lo = v/c Dl = wavelength shift lo = wavelength if source is not moving v = velocity of source c = speed of light

11. Frequency and wavelength are intimately related for a wave. How often peak passes finish line Distance between peaks Finish line How fast wave moves to right

12. Blackbody

13. 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 Photon enters If hole is very small, what is probability that it exits?

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

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

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

17. 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

18. 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

19. Watt? Energy Flux?

20. 2000 Calories (unit of energy) over 24 hours is about 100 Watts 100 Watt light bulb

21. Watt? Energy Flux? • A Watt is a unit of Energy [Joule] per time [second]. Joule is related to Calorie, which is a unit of energy we use for humans. • For example, your electricity bill tells you how many kilowatt-hours you used. If you use 1 kilowatt for 100 hours then you used 100 kilowatt-hours

22. Watt? Energy Flux? Blue photon 1 meter x 1 meter square We also use Watts/(m^2). If you have a 1 meter square solar panel and it is being hit by 1 blue photon per second, you can compute the energy flow into the solar panel. Remember that a Watt is a Joule/second, and each photon has a certain amount of energy in Joules that is given by Plank’s law.

23. 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.

24. Blackbodies and Astronomy

25. 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

26. 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 • The intensities of radiation emitted at various wavelengths by a blackbody at a given temperature are shown by a blackbody curve

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

28. 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:

29. Special Relationship Think of total fluxas related to area under this curve*. Add up contribution from each wavelength Stefan-Boltzmann Law tells us that if we add up the energy flux from all wavelengths, then the total energy Flux Energy Flux Intensity Wavelength * But not identical, so area does not scale by T4. The area under a similar-looking curve does scale with T4

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

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

33. 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

34. Color and Temperature

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

37. Why does it glow white before blue?

38. Can this figure help us explain?

39. 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?

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

41. http://www.shodor.org/refdesk/Resources/Models/BlackbodyRadiation/applet.htmlhttp://www.shodor.org/refdesk/Resources/Models/BlackbodyRadiation/applet.html http://casa.colorado.edu/~ajsh/colour/Tspectrum.html

42. 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)

43. 5 A B 4 C 3 Energy Flux 2 1 0 One curve is ideal blackbody, one is measured above Earth’s atmosphere, one is measured at Earth’s surface.

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

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

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

47. 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. What would happen to the dips in C?

48. 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

49. 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

50. Cloud of gas is like Earth’s atmosphere