Understanding Electromagnetic Radiation and Thermal Radiation

1. Lecture 8 Phys 1810 • Read BEFORE coming to class: • Electromagnetic Radiation 3.1 to 3.4 • Energy • Thermal Radiation Box 3-2 • Flux and Luminosity (L equation in Box 17-2) • Spectra 4.1, 4.2 • Kirkhhoff’s Laws • Radio Emission 18.4 • Doppler shift: 3.5, Box 3-3, 4.5 • Telescopes 5.2, 5.3, “seeing” in 5.4, 5.5-5.7 Password! Given out in Class, not email. TODAY! Office hour 3pm Allen 514 To do practice questions for test/exam, the textbook online code is required. The class lecture website is http://www.physics.umanitoba.ca/~english/2014fallphys1810/

2. [EM]

4. Electromagnetic Wave summary Text Recall column Text • oscillations occurring perpendicular to the direction of energy transfer • oscillating electric & magnetic fields

5.  a B field accompanies a changing E field. Thus a vibrating charged particle in a star create EM waves in its own EM field and these waves propagate through space.

6. Hole in wall

7. Hole in the Wall Expected for particles Observed

8. Check the class website for videos! What happens if you send photons one at a time through a double slit? Double Slit  Interference Pattern • Would you get only 2 strips as if the photons were “baseballs” ? • https://www.youtube.com/watch?v=MbLzh1Y9POQ http://www.olympusmicro.com/primer/java/doubleslitwavefronts/index.html Demonstrates the DUAL NATURE of light.

9. Particle Description  Photons

10. Photon Energy (E) h== Planck constant. but So Higher frequencies have higher energies How does the speed of radio waves compare to the speed of visible light? • They both travel at the same speed.

11. You may want to write down what I say about each range and transparency.

13. Thermal Radiation summary Text Recall column Text • “heat” • most familiar kind of radiation • caused by random motions of atoms & molecules • a lot of energy available large amount of motion (high temperature) T== temperature

14. Blackbody Radiation summary Text Recall column Text • blackbody (b.b.) radiation is thermal radiation emitted a blackbody • blackbody == “perfect absorber” & re-emits radiation in all directions! (doesn’t scatter) • no “perfect” blackbody but close: • some ovens • stars (sun) • cosmic background radiation

15. Temperature Scales summary Text Recall column Text

16. Blackbody Radiation summary Text Recall column Text • b.b.semit across a range of λ • but intensity not the same at all λ • Temperature (T) of b.b. determines intensity of radiation & the peak λ

17. Blackbody Radiation summary Recall column Intensity Intensity b.b. radiation depends only on its T. The intensity changes at different wavelengths. (Graph of 1 object at 1 specific T.) Wavelength 

18. Blackbody Radiation: b.b. curve summary Recall column Explained using particle theory of light photons of energy Intensity Intensity b.b. radiation depends only on its T. X-axis Book uses increasing frequency (nu). Most astronomers use increasing wavelength (lambda), so we’ll use this. Wavelength  Frequency 

19. Blackbody Radiation: Wien’s Law summary Wavelength  Recall column Intensity Peak Intensity

20. 20,000° K 10,000° K 5000° K Intensity 2000° K 1000° K 500° K Wavelength (nm) X-Ray Ultraviolet Visible Infrared Microwave Radio Blackbody Radiation Curves for Different Temperatures summary Text Recall column

21. Example using Wien’s Law: .

22. E.g. If is at short wavelengths for a b.b., then its T is higher & the object’s emission is towards blue end of EM spectrum.

23. Star A Star B Intensity Star C Star D Star E Wavelength Short Long summary Text Recall column A plot of blackbody spectra of five different stars is shown in the figure. Based on these spectra, the star with lowest T is Star E.

24. What colour does this star have? summary Recall column Red

25. summary Recall column (greenish) Yellow-white

26. summary Recall column Blue

27. what is hot & what is not? summary Recall column • The hottest stars in this image appear: • Blueish • Reddish

28. Contrast with everyday experience! summary Recall column

29. On supplemental page. summary Examples of Blackbodies and their Temperatures. Recall column

30. Objects and Peak of Emission Dense, spherical clouds: radio and Far-IR Globule of dust: IR Sun: visible White dwarf star/planetary nebula: UV

31. Flux • related to temperature (T) Stefan-Boltzmann law for b.b: Relates T to the total amount of energy (E) that the b.b. emits at all wavelengths.

34. Balloon & Surface Area • 2 stars with the same T but different surface area. • Total energy output over the whole sphere of larger object is larger.

35. To here for the afternoon

36. Stars: Their Characteristics • Luminosity (L): The total energy radiated per second, at all wavelengths. • L = surface area * flux • Surface area of a sphere is T== Surface Temperature  Luminosity is proportional to the radius squared times surface temperature to the 4th power.

37. Stars: Why Temperature is useful. • Notice that if we know the temperature of a star, then if we know the radius, we can calculate the luminosity. • Alternatively, if we know the temperature and the luminosity we can determine the radius. 

39. The Interaction of light and matter. summary Recall column • Photons (γ == gamma) • Individual packet of EM energy that makes up EM radiation • γ & matter interact creating spectra. • Spectra used to assess • T (blackbody curve type spectrum) • processes that produce light or absorb it (i.e. what is going on) (Animation)

40. Spectra Kirchhoff’s Laws summary Recall column • 3 empirical laws • Hot opaque body -> continuous spectrum • Cooler transparent gas between source& observer -> absorption line spectrum • Diffuse, transparent gas -> emission line spectrum

41. Spectra summary Recall column • This kind of spectrum (continuum) is caused by • Hot, low density gas • Hot, dense blackbody • Cooler transparent gas

42. Spectra • Our sun and other stars have an atmosphere. Imagine that you are in a spaceship far above the Earth’s atmosphere. Which of the following spectra would you observe when analyzing sunlight? • Continuum rainbow-like spectrum • Dark line absorption spectrum • Bright line emission spectrum

43. Spectral Finger Prints Solar Spectrum • Note emission lines for lab spectrum of iron are at same λs of absorption lines of iron in  • Can use line spectra to determine chemical elements in object.

44. Interaction of Light and Matter: How are line spectra created? γs of light interact with atoms & molecules. • Atoms consist of: • Electrons (negative charge) == e- • Nuclei (balance charge of e-) • Protons (positive) • Neutrons (neutral) • Molecules: group of 2 or more atoms.

45. Interaction of Light and Matter • Hydrogen == H: simplest atom. • 1 e- & 1 proton. • Classical picture: e- in an orbit. • Contemporary picture: e- as a cloud. • Orbits are really energy levels. • E == energy

46. Interaction of Light and Matter Hydrogen Atom Energy Levels • Every chemical element has its own specific set of E levels. • Each E level is associated with a λ.

47. Interaction of Light and Matter Creating spectral lines at visible wavelengths • specific (quantized) E levels. Level with lowest E is ground state. • How does e- get excited? • By interactions between γs & matter.

48. Interaction of Light and Matter: Creating spectral lines at visible wavelengths • The e- can shift between E levels by absorption & emission of γs.

49. To here for the morning.