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Light

Light. Radiation and Spectra Chapter 5. What is Light?. Newton Prism shows white light contains all colors Light made of particles (photons) Maxwell Theory of electricity and magnetism Light is electromagnetic waves Produced by wiggling electrons Radiation = production of light

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Light

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  1. Light Radiation and Spectra Chapter 5

  2. What is Light? • Newton • Prism shows white light contains all colors • Light made of particles (photons) • Maxwell • Theory of electricity and magnetism • Light is electromagnetic waves • Produced by wiggling electrons • Radiation = production of light • Quantum Mechanics • Light is both: particle and wave

  3. Waves • Wavelength ( l) • Distance between crests (or troughs) • Frequency ( f ) • How often it repeats (wiggles up and down) • Measured in Hertz (Hz) • number of times per sec

  4. Waves • Speed c = 3 x 108 m/s c = lf • Wavelength inversely related to frequency l = c / f • high frequency = short wavelength • low frequency = long wavelength

  5. Particles as Waves • “Wave Packet” • particle/photon = localized wave

  6. Properties of Light • Color • Depends on frequency • blue = high frequency = short wavelength • red = low frequency = long wavelength • Carries energy (heat) • Photon energy E = h f • high frequency = high energy = blue • low frequency = low energy = red h = Planck’s constant

  7. Red light has ____ than blue light. A. larger frequency, energy, and wavelength B. smaller frequency, energy, and wavelength C. larger frequency and energy, but smaller wavelength D. smaller frequency and energy, but larger wavelength

  8. Which of the following travels fastest? } radio waves infrared (heat) waves microwaves blue light waves none of the above All are types of light! All types of light travel at the same speed - the “speed of light”, c

  9. Propagation of Light • Photons travel in straight lines • energy spread over larger area at larger distances • produces 1/r2 decrease in brightness • Double distance - brightness decreases by 4

  10. If a 100-watt light bulb is placed 10 feet away from you, and an identical 100-watt light bulb is placed 100 feet away from you, which will appear brighter? The closer one The farther one They will appear the same brightness How much fainter will the far one appear compared to the close one? Twice as faint 10 times fainter 100 times fainter 1000 times fainter ~ 1/r2

  11. Electromagnetic Spectrum • Visible light: • red, orange, yellow, green, blue, indigo, violet (ROYGBIV) • Invisible Light: • Ultraviolet = bluer than blue • Infrared = redder than red • Other wavelengths: • Short: X-rays, gamma-rays • Long: microwave, radio

  12. Thermal Radiation • All objects radiate (thermal radiation) • Objects made of atoms • Atoms (and their electrons) vibrate • Wiggling electrons radiate, producing light • Bigger objects produce more light • Higher temperature = stronger vibration • Hotter objects emit more light • Perfect absorber is black • Absorbed light (energy) heats object • Temperature increases until emitted energy = absorbed energy • Emitted radiation called Blackbody Radiation • Thermal radiation emitted by most objects similar to blackbody

  13. Blackbody Radiation Laws • Luminosity, L L = energy emitted per second • Luminosity for a spherical object (a star) L = 4p R2 sT4 R = radius (size) of star; T = temperature • double size, luminosity increases by 2x2 = 4 • double temperature; luminosity increases by 2x2x2x2 = 16 Stefan-Boltzmann Law

  14. WORKBOOK EXERCISE: Luminosity Temperature and SizePart I (pages 33-35 in workbook)

  15. Blackbody Radiation

  16. Blackbody Radiation Laws • Color • Wavelength where most light emitted lmax = 3 x 106 / T T in Kelvin; lmax in nanometers (1 nm=10-9m) • Cool stars are red • Hot stars are blue • Color indicates temperature! Wien’s Law As T , Wavelength , Color = redder As T , Wavelength , Color = bluer

  17. Homework: WORKBOOK EXERCISE Blackbody Radiation (pages 37-40 in workbook)

  18. The graph above shows blackbody spectra for three different stars. Which of the stars is at the highest temperature? • Star A • Star B • Star C Because peak energy emission occurs at shortest wavelength

  19. Doppler Shift • Originally discovered using sound waves • Moving object • emits light with slightly different color • Frequency (pitch) of approaching object is higher • Blueshift • Wavelength shorter (shifted blueward) • Frequency (pitch) of receeding object is lower • Redshift • Wavelength longer (shifted redward)

  20. Doppler Shift Redshift Blueshift

  21. Spectroscopy • Prism separates light into different colors • Continuous spectrum • contains all colors • Example: blackbody spectrum

  22. Spectroscopy Absorption Line Spectrum • Some colors are missing (discrete lines) Solar Spectrum N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF

  23. Spectroscopy • Emission Line spectrum • Only certain colors are present (discrete lines) • Spectrum for each element unique (like fingerprints)

  24. Model Atom • Nucleus • contains protons and neutrons • number of protons = element (1 proton = hydrogen, 2 protons = helium, etc.) • number of neutrons about same as protons • Isotope = different number of neutrons hydrogen helium Isotopes of hydrogen

  25. Model Atom • Electrons orbit nucleus • Number of electrons = number of protons • Ionization = removing electrons • Only certain orbits are allowed hydrogen helium

  26. Atomic Absorption • Atom absorbs photon energy • electron “jumps” to higher energy orbit • only certain discrete orbits are allowed • Atom can absorb only discrete colors(energies)

  27. Atomic Emission • Electron “jumps” to a lower energy orbit • Atom emits photon • can emit only discrete colors • same colors (wavelengths/energies) as absorption

  28. Atomic Energy Levels • Energy Levels • Different for each element • each element has unique set of absorption/emission lines

  29. Kirchoff’s Laws • Continuous spectrum • Produced by hot solid (or dense gas) • Emission line spectrum • Produced by hot, low density gas • Absorption line spectrum • Produced when continuous source is viewed through cooler low density gas

  30. Kirchoff’s Laws • Absorption lines same wavelengths as emission lines • Gas can only absorb and emit at certain discrete frequencies/wavelengths/energies

  31. WORKBOOK EXERCISE: “Types of Spectra” (pages 41-42 in workbook)

  32. If you analyze the light from a low density object (such as a cloud of interstellar gas), which type of spectrum do you see? • dark line absorption spectrum • bright line emission spectrum • continuous spectrum

  33. Imagine that you observe the Sun while in your space ship far above Earth’s atmosphere. Which of the following spectra would you observe by analyzing the sunlight? • dark line absorption spectrum • bright line emission spectrum • continuous spectrum

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