Light

1. Light Solar System Astronomy Chapter 4

2. Light & Matter • Light tells us about matter • Almost all the information we receive from space is in the form of light. • The light can tell us the conditions of objects in space – temperature, composition, motions, etc. • Light has many strange properties which stretch our ideas of what is “real.”

3. Light: Wave or Particle? • Light can be both like waves and like particles (photons) • The particle picture is helpful when thinking about how light is absorbed and emitted • The wave picture is best for describing how light gets from one place to another

4. Light as a Wave • Light is a wave of electric and magnetic fields – electromagnetic (EM) wave • The wavelength (λ) is the length between crests of the wave • The frequency (f) is the number of waves that pass by each second • Different types of light (visible, infrared) have different wavelengths

5. Wavelength & Frequency

6. Wavelength, Frequency, & Speed • Speed of light in a vacuum • 3×108 m/s (300,000 km/s) • Travels more slowly through materials like glass or water • Wavelength and frequency are related:

7. Visible Light • Our eyes see a small range of EM radiation • Red light λ = 700 nm • Violet light λ = 400 nm • Spectrum: • ROY G BIV

8. Visible Light • Our eyes see a small range of EM radiation • Red light λ = 700 nm • Violet light λ = 400 nm • Spectrum: • ROY G BIV

9. Electromagnetic Spectrum • Visible light is just a small part…

10. Electromagnetic Spectrum • Visible light is just a small part…

11. Hydrogen Spectrum • A Big Mystery

12. Kirchoff’s Laws of Radiation

13. Light as a particle • “Photons” • Little packets of energy • Atoms can absorb or emit • Photons can carry different amounts of energy • High energy • short wavelength • high frequency • Low energy • long wavelength • low frequency

14. Atoms & Light • Neils Bohr • Electrons surround/orbit nucleus • can have certain energies; other energies are not allowed. • Each type of atom (carbon, oxygen, etc.) has a unique set of energies. • A good way to represent the energies is with an energy level diagram.

15. Atoms & Light Explained hydrogen spectrum precisely!

16. Atoms & Light • Spectroscopy

18. Atoms & Light • Mystery Gas…

19. Doppler Effect

20. Doppler Effect

21. Doppler Effect – Radial Velocity • Radial velocity is part toward or away from observer • Along line of sight • Toward gives shorter wavelengths – redshifted • Away gives longer wavelengths -- blushifted

22. Doppler Effect

23. Doppler Shift – Concept Quiz • Hydrogen emits light at λ = 656 nm. You see a distant galaxy in which the light from hydrogen has λ = 696 nm. Is this galaxy … • Moving toward us? • Moving away from us?

24. Light & Temperature Solar System Astronomy … still Chapter 4

25. Emitted Light • All objects emit light (or EM radiation) • What kind depends on temperature and state (solid or gas) • Light carries off energy • Rate of loss is called luminosity

26. Energy Balance • Planet’s temperature is a balance • Energy absorbed from sun • Energy emitted from planet • From temperature • Temperature is a measure of heat radiated • Balance is example of thermal equilibrium • Very important concept in many areas of astrophysics

27. Temperature • All atoms are constantly in thermal motion • Temperature is a measure of average speed (kinetic energy) of atomic motion • Measure in Kelvin • Water freezes/boils 273 K/373 K • Minimum possible 0 K • Sun is 5800 K (10,000˚ F)

28. Temperature • Measure of energy/motion • More temperature is more motion • More pressure increases motion and temperature • Temperature & pressure are CLOSELY linked

29. Blackbody Radiation • Dense objects emit radiation • Blackbody radiation • Thermal radiation • Continuous radiation/spectrum • For two objects of same size • Hotter emits more light at all wavelengths • Emit more total energy per second (higher luminosity) • More of the radiation is at shorter wavelengths

30. Blackbody Radiation

31. Blackbody Radiation

32. Blackbody Radiation • Of note: • Some light is emitted at all wavelengths • Often a negligible amount • Little very short or very long wavelengths • There is a peak wavelength

33. Stefan’s Law • Flux is the total energy emitter per area (m2) • Hotter objects emit MUCH MORE ENERGY

34. Wien’s Law • Temperature relates to λpeak • Hotter means bluer • Simple measurement to calculate temperature

35. Brightness • Amount of light that arrives at a particular place • Inverse-square law

36. Equilibrium Temperature • Equilibrium reached when • Energy absorbed equals energy emitted • Distant planets are cold mainly because of inverse-square law • Actual temperature depends on how well planet absorbs incoming light • albedo

37. Equilibrium Temperature

38. Concept Test • If the Sun got hotter, which of the following would be true? • The flux from the Sun would increase • The peak of its spectrum would shift to redder colors • The brightness at the Earth would decrease

39. Concept Test • Compared to the brightness of the Sun at the Earth, the brightness at ½ AU would be • ¼ as much • ½ as much • The same as now • Twice as much • Four times as much

40. Concept Test • In the distant future the Sun will be cooler but will emit far more energy every second than it does now. What will happen to the Earth’s temperature? • It will be hotter • It will be the same • It will be cooler