1. Astronomy Picture of the Day

2. Radiation and Spectra Chapter 5
Light
Radiation and Spectra
Chapter 5

3. What is Light? Newton Maxwell Quantum Mechanics
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

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

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

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

7. Properties of Light Color Carries energy (heat) 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

8. 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

9. All types of light travel at the same speed -
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

10. CPS Question The color of visible light is determined by its ____.
A) brightness
B) amplitude
C) speed
D) wavelength

11. CPS Question
If the wavelength of light increases, the frequency must ____.
A) increase also
B) decrease
C) remain unchanged

12. CPS Question
The bending of light that occurs when moving between media of different densities is called ___.
A) reflection
B) refraction
C) diffraction
D) distortion

13. 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

14. 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

15. The Electromagnetic Spectrum
1 nm = m , 1 Angstrom = m
c =
f

16. 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

17. Which kind of electromagnetic radiation has a
wavelength longer than that of visible light?
A. infrared
B. ultraviolet
C. x-rays
D. gamma rays
E. none of the above

18. What’s the wavelength of my favorite radio station?
Link to:

19. 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

20. What does the spectrum of an astronomical object's radiation look like?
Many objects (e.g. stars) have roughly a "Black-body" spectrum:
Brightness
Frequency
also known as the Planck spectrum or Planck curve.

21. Blackbody Radiation Laws
Luminosity, L
L = energy emitted per second
Luminosity for a spherical object (a star)
L = 4p R2 s T4
R = radius (size) of star; T = temperature
double size, luminosity increases by 2x2 = 4
double temperature; luminosity increases by 2x2x2x2 = 16
Stefan-Boltzmann Law

22. Spectroscopy and Atoms
How do you make a spectrum?

23. When you bend light, bending angle depends on wavelength, or color.
Refraction of light
When you bend light, bending angle depends on wavelength, or color.

24. Questions How is temperature related to the amount of energy radiated?
How is temperature related to the color of the object?
(Blackbody Demo)

25. The wavelength of peak emission tells us the temperature of the object!
"cold" dust
"cool" star
Sun
"hot" stars
frequency increases, wavelength decreases

26. Blackbody Radiation

27. 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

28. The graph above shows blackbody spectra for three different stars
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

29. 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)
video
Video

30. Doppler Shift
Redshift
Blueshift

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

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

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

34. Pattern of lines is a fingerprint of the element

35. Sodium emission and absorption spectra
For a given element, emission and absorption lines occur at the same wavelengths.
Helium discovered in Sun’s spectrum before being found on Earth!
Sodium emission and absorption spectra

36. Spectrum of the Sun Absorption spectrum
What causes emission/absorption of light at specific wavelengths?
Interactive
Video 1, 2, 3

37. Types of Spectra
1. "Continuous" spectrum
2. "Emission" spectrum
3. "Absorption” Spectrum
video

38. The Particle Nature of Light
Light interacts with matter as individual packets of energy, called photons. c
photon energy is proportional to frequency:
E f (or E 
example: ultraviolet photons are more harmful than visible photons. 1 

39. 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

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

41. The Nature of Atoms (Fair Analogy)
The Bohr model of the Hydrogen atom:
electron _ _ + +
proton
"ground state"
an "excited state"
(Fair Analogy)

42. 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)

43. When an atom absorbs a photon, it moves to a higher energy state briefly
When it jumps back to lower energy state, it emits photon(s) in a random direction, conserving the total energy of the system!

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

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

46. Other elements Helium Carbon
neutron proton
Each element has its own allowed energy levels yielding a unique spectral fingerprint.

47. Kirchoff’s Laws Continuous spectrum Emission line 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

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

49. 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

50. 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

51. CPS Question
Which ONE of these is constant for all forms of EM radiation in a vacuum?
A) amplitude
B) wavelength
C) frequency
D) speed
E) energy

52. CPS Question Which ONE is NOT a property of a blackbody?
A) It appears black, regardless of its temperature.
B) It emits radiation in a continuum of wavelengths.
C) Its spectrum peaks at a wavelength determined by its temperature.
D) The total energy that it radiates increases rapidly with temperature.

53. CPS Question The Sun's observed spectrum is _____.
A) A continuum with no lines, like the rainbow.
B) A continuum with bright emission lines.
C) Only absorption lines on a black background.
D) Nearly a continuum with some absorption lines.

54. Ionization Hydrogen _ _ + + _ _ Helium + + + + _ _ "Ion"
Absorbing a high energy photon and atomic collisions can both lead to ionization.

55. Spectrum of the Sun Complicated objects => many different elements
Nearly continuous absorption spectrum
What causes emission/absorption of light at certain wavelengths?

56. . . . . . . Why emission lines? hot cloud of gas
- Photon absorption/atomic collisions excite atoms
- Electron drops back to lower level
- Photons at specific frequencies emitted

57. (Shockwave Demo) (Web Link)
Why absorption lines? . . . . .
cloud of gas . . . . . .
(Shockwave Demo) (Web Link)

58. Fusion generates continuous spectrum
Stellar Spectra
Sun's 'atmosphere' absorbs specific wavelengths
Fusion generates continuous spectrum
Star

59. Kirchhoff's Laws
1. Continuous spectrum
2. Emission spectrum
3. Absorption spectrum

60. Question
How does the pitch or tone of a sound wave change when the source of the sound is moving towards or away from you?
What about when you are moving towards or away from the source?
Does this effect occur for all types of waves or just for sound waves?

62. Doppler Shifted Atomic Spectra
Why don’t we see the color of everyday objects change as they move?

63. We've used spectra to find planets around other stars! (Ch. 4)

64. Star wobbling causes Doppler shift of its absorption lines.
Only gives information about velocity along line of sight!