Light: The Astronomer’s Tool

1. Light: The Astronomer’s Tool

2. Review of Last Time • In most cases, light is the only thing that astronomers can use to study the universe • An understanding of light is very important to understanding astronomy

3. Review of Last Time • Light can be thought of as a wave of electromagnetic energy • Demonstrated by interference; polarization by magnetic fields • Frequency and wavelength are important

4. Review of Last Time • Light can also be thought of as a particle • Demonstrated by photo-electric effect • Speed of light is constant • Time and distance are relative • Can be used to measure distance

5. Review of Last Time • Important Equations • Who can remember them?

6. This Time • How is light produced? • What can astronomers learn from light? How? • How do astronomers collect light?

7. How is Light Made? • We all can think of things that give off of light • What we want to know is “Why?” • What do you think?

8. How is Light Made? • Remember that light is an electromagnetic phenomenon • Light production must be related to electricity and magnetism • It turns out that any accelerating charge produces photons

9. How is Light Made • Let’s dissect that statement • Accelerating charge –a change in velocity (either speed or direction) • This means we need a force • Produces photons • Easiest to think of particles in this case

10. How is Light Made? • What force is most likely to be at work here? • The Electromagnetic force

11. The Electromagnetic Force • This should look familiar • Very similar to gravity • Instead of mass, have charge • Instead of G, have k • Some things to remember • Opposites attract (negative sign) • Likes repel (positive sign) • 1/d2

12. The Electromagnetic Force • Since the EM force causes charges to accelerate, and this produces photons, photons are sometimes called “messenger particles” for the EM force

13. The Next Question… • We now know that accelerating charges produce photons • What are some situations that this might occur in? • Since electrons are the most common charge carriers, we should think about them

14. Electrons and Light • Where is one place we are likely to find electrons? • In atoms

15. Atomic Structure • We often think of the electron as orbiting the nucleus like a planet

16. 0 of 5 Is the electron in this picture accelerating? • No, there is no change in its speed • Yes, it is constantly changing direction

17. Atomic Structure • How is the EM force working here? • The positively charged nucleus attracts the negatively charged electrons • Circular motion guarantees acceleration because direction is always changing • Electron is continuously radiating

18. Be Careful… • But there is a colossal problem with this picture • Think about…what will happen to this accelerating charge?

19. Be Careful… • Accelerating charge will emit photon • Photon will carry away energy equal to h*n • Energy must be conserved…electron must lose energy • Without as much energy, electron will move closer to nucleus

20. Collapsing Atoms • This is catastrophic • The electron will just continue to spiral into the nucleus • This will only take about xxx seconds!!! • But we are here and atoms exist, so something must be wrong

21. Quantized Energy • There is only one solution • Different laws of physics are at work in atoms • Quantum Mechanics • Niels Bohr proposed that electrons can only have discrete, quantized amounts of energy

22. Energy Levels • The Bohr Model only allows the electrons to exist in certain, specific energy levels • There is a minimum energy level…the electrons can’t get closer than this • Stops electrons from spiraling in

23. The Bohr Model

24. Energy Levels • At first this may seem odd, but there are analogous things in every day life

25. Energy Levels

26. Energy Levels • Why don’t we experience this quantized nature of the universe in our lives? • On larger scales, energy levels “blur together” and are practically continuous

27. Getting Back To Light… • What does this have to do with light? • Bohr was not just trying to explain how atoms can exist • Also wanted to explain line spectra

28. 0 of 5 Why do different elements have different spectra? • Because there is less gas in the tubes • The current passing through each tube is different • Each element has a different atomic structure

29. Line Spectra • How does the Bohr model explain what we just saw? • Electrons are changing energy levels and releasing photons in the process

30. Line Spectra

31. Line Spectra • We know that photons are being emitted, because we see the bright lines • This means the photons must be carrying away energy from something • How much energy?

32. Line Spectra • Where does this energy come from? • From electrons, who move from a higher to a lower energy level • By conservation of energy, the difference in energy between these two levels must be

33. Absorption Lines • So far we have talked about what happens when electrons lose energy by dropping to lower energy levels • Can you predict what will happen if an electron moves to a higher energy level? Let’s try to explain the process and consequences from beginning to end

34. Absorption Lines • If electron is moving to higher energy level, it has to gain energy • This energy must come from somewhere, and it comes from a photon • In other words, an electron absorbs a photon • There are now fewer photons at this wavelength • We see a dark band at this wavelength, called an absorption line

35. 0 of 5 Absorption lines will occur at the same wavelengths as their corresponding emission lines. • True • False

36. Absorption Lines • Since the difference in energy levels is the same regardless of whether the electron loses or gains energy, the wavelength of emission and absorption are the same

37. A Key Point • As we saw in the demo, different elements and compounds had different emission spectra • This is due to their different atomic structures

38. Atomic Fingerprints • The wavelength of the emission (or absorption) line depends on the difference between the energy levels in the atom • Each element and compound has a unique atomic structure with unique energy levels • Therefore, the wavelengths of emission and absorption are unique “fingerprints” of any atom or molecule

39. Emission and Absorption Spectra • The last point is extremely important • If we can obtain a spectra of any object, we can determine what that object is made of • Here are some examples of the spectra

40. Emission Lines

41. Emission Lines

42. Absorption Lines

43. Absorption Lines

44. Emission and Absorption Spectra • And here is an example of how we determine the elements that are present

45. Continuous spectra • So far we have talked about line spectra • Emission at only specific wavelengths • Can something emit light over a wide range of wavelengths • Yes, and we call this a continuous spectrum

46. Continuous Spectra • Here is the idea… • Dense objects (i.e. not low density gas) have a complex atomic-scale structure • Individual atoms interact with, and are affected by the atoms near them • The result is a more complex system of energy levels, and a more complex way of filling them

47. Energy Level Population • At any given time, some electrons might have low energy, some might have high energy, and some may be in between • Just how many electrons are in each group will determine the wavelength of most of the radiation • But we will still get light at other wavelengths

48. Energy Level Population • Scientists talk about the number of electrons in each energy level in terms of the “energy level population” • To predict what the spectrum of an object will be like, we need to describe the nature of the energy level populations

49. The First Attempt • This was a major area of research at the end of the19th century • Scientists had not yet begun to think of light as a particle

50. The Ultraviolet Catastrophe • There was a problem • In their mathematical descriptions, they predicted that a warm “oven” should emit an infinite amount of power • Obviously impossible