1. What is light? Light is electromagnetic radiation.
Technically, light is the part of electromagnetic (e.m.) radiation that humans (and other animals) see.
Although incorrect, we usually call “light” all types of electromagnetic radiation, like X-ray light or UltraViolet light
Light is made of energy.
Light always travels at the speed of light: c.
Light is a wave. 1

2. What is Electromagnetic Radiation?
Made of propagating waves of electric and magnetic fields.
It carries energy with it
Sometimes called “radiant energy”
Think – solar power, photosynthesis, photo-electric cells, the fireplace … 2

3. What is an electromagnetic wave?
It is electricity and magnetism moving through space.
Electric magnetic force
So, when we say the speed of light is “c” what we really mean is that the speed of the electromagnetic wave is “c”, regardless of its frequency. 3

4. Light is an electromagnetic wave
Light as a wave
Waves you can see:
e.g., ocean waves
Waves you cannot see:
sound waves
electromagnetic waves
Pitch and intensity
Light is an electromagnetic wave 4

5. Properties of Waves For light in general: speed = c = d/t = λ 
Wavelength – the distance between crests (or troughs) of a wave.
Frequency – the number of crests (or troughs) that pass by each second.
Speed – the rate at which a crest (or trough) moves.
For light in general:
speed = c = d/t = λ 
λ  = c
λ = c/
wavelength
frequency
speed of light = 3x108 m/s
in vacuum
Boxcars and speed 5

6. Light as a Wave
Wavelengths of light are measured in units of nanometers (nm) or Ångström (Å):
1 nm = 10-9 m
1 Å = m = 0.1 nm
Visible light has wavelengths between 4000 Å and 7000 Å (= 400 – 700 nm). 6

7. Different colors of visible light correspond to different wavelengths.
Wavelengths and Colors
Different colors of visible light correspond to different wavelengths. 7

8. Visible Light
Shorter
Wavelength
Longer
Wavelength 8

9. Remember: visible light isn’t the whole story
Remember: visible light isn’t the whole story. It’s just a small part of the entire electromagnetic spectrum
Short Wavelength
Long Wavelength
(high frequency)
(high energy)
(low frequency)
(low energy) 9

10. Wavelengths and size of things
Roy G. Biv 10

11. Reminder #1: E.m. Radiation generally contains bundles of waves of different wavelengths (colors)
How much of each color is present in a given bundle of e.m. radiation, i.e. the distribution of intensity of each wavelength, is called the spectrum
Here is an example of optical (visible) light:

12. The Multi-wavelength Sun
infrared
Radio
X-ray
optical 12

13. Optical Sky 13

14. Near-infrared sky
Boldt et al. 14

15. Radio Sky 15

16. Soft X-ray Sky 16

17. Different wavelengths carry different types of information
Visible light: the glow of stars (dust blocks the light)
Infrared: the glow of dust
Visible light (top) and infrared (bottom) image of the Sombrero Galaxy 17

18. Light as particles
Light comes in quanta of energy called photons – little bullets of energy.
Photons are massless, but they have momentum and energy.
Couldn’t explain things if light is always a wave. Gasoline vs. apple 18

19. Wave-particle duality
All types of electromagnetic radiation act as both waves and particles.
The two views are connected by the relation
E = h = h c / 
h is the Planck's constant, c is the speed of light
is the frequency, is the wavelength
You can't keep dimming down light of a given color (say, red) indefinitely and see a steady red signal.
Double the frequency and E doubles, etc
The energy of a photon does not depend on the intensity of the light!!! 19

20. Intensity
A photon's energy depends on the wavelength (or frequency) only, NOT the intensity.
But the energy you experience depends also on the intensity (total number of photons). 20

21. In Summary: properties of Light
All light travels through (vacuum) space with a velocity = 3x108 m/s
The frequency (or wavelength) of a photon determines how much energy the photon has (E=h).
The number of photons (how many) determines the intensity.
Light can be described in terms of either energy, frequency, or wavelength. 21

22. Compared to visible light, radio waves have:
higher energy and longer wavelength
higher energy and shorter wavelength
lower energy and longer wavelength
lower energy and shorter wavelength
all light has the same energy 22

23. Origin of light
Light (electromagnetic radiation) is just varying electric and magnetic fields that propagate through space.
Now, two very important things happen in nature:
An electric field that varies in strength (e.g., owing to the acceleration of an electron) generates electromagnetic radiation.
Electromagnetic radiation, in turn, accelerates electrons (or any electrically charged particle)
We discuss two major mechanisms of light production:
Blackbody Radiation, a.k.a. thermal radiation
Spectral Line Emission by atoms and molecules

24. Heat and Temperature
Temperature refers to the degree of agitation, or the average speed with which the particles move (T~v2).
All atoms and molecules are moving and vibrating unless at absolute zero
Temp = -273 C = 0 K = F
Heat refers to the amount of energy stored in a body as agitation among its particles and depends on density as well as temperature.
Things are composites of molecule, which are made of atoms. They are bound together by the e-M force…

25. Scale of Temperature Fahrenheit Centigrade Kelvin F C K 212
Water Boils
100
Water Boils
373 32
Water Freezes
Water Freezes
273
-459.7
Absolute Zero
-273
Absolute Zero

26. Scale of Temperature C = (F-32) / 1.8 F = 1.8*C + 32
C = K K = C
Absolute zero of temperature (atoms are still)
K = 0 C = F =

27. Blackbody Radiation
Acceleration or deceleration of an electron results in the production of an electromagnetic wave.
Heat makes electrons and atoms move: thermal motion.
Electrons collide against atoms and each other all the time, i.e. accelerate and decelerate all the time.
Makes electromagnetic radiation so also called thermal radiation.
Makes a continuous spectrum of radiation so also called continuum radiation.

28. Blackbody Spectrum
It is the spectrum of radiation from thermal motions of matter.
Temperature is how fast the electrons and atoms are moving on average.
Faster motions means more energetic collisions and higher energy photons.

29. Hotter objects have electrons moving with higher speed, thus they emit photons with a higher average energy.
Wien’s Law:
The wavelength at the peak of the blackbody emission spectrum is given by
max (nm)= 2,900,000/T
(P.S. remember that E = hc/)
Thus, the hotter the matter, the higher the energy of the e.m. radiation
Introduce temperature units, thermometer

31. Total emitted power: E = 4  R2 σ T4
Hotter objects emit more total radiation per unit area. However, a big cold object can emit the same or more energy (depending on how big it is) than a small, hotter one
Cold
Hot
Stefan-Boltzmann Law:
Emitted power per square meter = σ T4
σ = 5.7 x 10-8 W/(m2K4)
Total emitted power: E = 4  R2 σ T4

32. L=A T4

33. The graph below shows the blackbody spectra of three different stars
The graph below shows the blackbody spectra of three different stars. Which of the stars is at the highest temperature? Which has the highest energy (energy is the total area under the curve)?
1) Star A
2) Star B
3) Star C A B
Energy per Second C
Wavelength

34. Reminder: Blackbody Radiation, i. e
Reminder: Blackbody Radiation, i.e. a continuum of wavelengths with a characteristic distribution of strengths

35. is bluer when hotter. is both is neither
You are gradually heating up a rock in an oven to an extremely high temperature. As it heats up, the rock emits nearly perfect theoretical blackbody radiation – meaning that it
is brightest when hottest.
is bluer when hotter.
is both
is neither

36. Campfires
Campfires are blue on the bottom, orange in the middle, and red on top.
Which parts of the fire are the hottest? the coolest?
As atoms get hotter, they wiggle faster and collide with each other harder ---> they emit more light and more wiggles per second = frequency goes up = wavelength goes down. Thus bluer.
Thus, as temperature goes up light gets stronger and gets blue.

37. More on Blackbody Radiation (a.k.a. Thermal Radiation)
Every object with a temperature greater than absolute zero emits blackbody radiation.
Hotter objects emit more total radiation per unit surface area.
Hotter objects emit photons with a higher average energy.

38. Color and Temperature Orion
Stars appear in different colors,
Orion
from blue (like Rigel)
Betelgeuse
via yellow (like our sun)
to red (like Betelgeuse).
If the spectra of stars are black bodies, then these colors tell us about the star’s temperature.
Rigel

39. Stars come in different colors

41. How to Make Light (Part 2): Line Emission/Absorption (light with discrete wavelengths)
Structure of atoms
Energy levels and transitions
Emission and absorption lines
Light scattering
Please pick up a diffraction grating on your way into class.

42. The Structure of Matter: Atoms
Atoms are made of electrons, neutrons, and protons.
The binding force: the attractive Coulomb
(electrical) force between the positively charged protons in the nucleus and the negatively charged electrons around the nucleus.
A Planetary Model of the Atom
A story about atoms
To understand the microscopic processes involved and to study another emission mechanism
As an introduction to elements

43. The Structure of Matter: Atoms
An atom consists of an atomic nucleus (protons and neutrons) and a cloud of electrons surrounding it.
Almost all of the mass is contained in the nucleus, while almost all of the space is occupied by the electron cloud. 43

44. Electron Orbits
Electron orbits in the electron cloud are restricted to very specific radii and energies.
r3, E3
r2, E2
r1, E1
These characteristic electron energies are different for each individual atom, i.e. element. 44

45. Energy Levels
In other words, the electron in a given orbit has the energy that corresponds to that orbit.
The higher the orbit, the higher the energy.
Different atoms have different energy levels, set by quantum physics.
Quantum means discrete!
Ladder
Gasoline vs apple

46. Spectral Line Emission
Collisions (like in a hot gas) can provide electrons with enough energy to change energy levels.
A photon of the difference in energy between levels is emitted when the electron quickly falls down to a lower level.

47. Energy Levels of a Hydrogen Atom
Different allowed
“orbits” or energy levels in a hydrogen atom.
Emission line spectrum
Absorption line spectrum

49. Spectral Lines of Some Elements
Argon
Helium
Mercury
Sodium
Neon
Spectral lines are like a cosmic barcode system for elements.

50. Atoms of different elements have unique spectral lines because each element
has atoms of a unique color
has a unique set of neutrons
has a unique set of electron orbits
has unique photons

51. Spectral Line Emission again
If a photon of exactly the right energy is absorbed by an electron in an atom, the electron will gain the energy of the photon and jump to an outer, higher energy level.
A photon of the same energy is emitted when the electron falls back down to its original energy level or of a different energy if it falls down to a different energy level.

52. Atomic Transitions: excitation of atoms
Remember that Energy = Wavelength = Colors
Eph = E3 – E1
An electron can be kicked into a higher orbit when it absorbs a photon with exactly the right energy.
Eph = E4 – E1
Wrong energy
The photon is absorbed, and
the electron is in an excited state.
(Remember that Eph = h*c/)
All other photons pass by the atom unabsorbed. 52

53. Atomic Transition: Excitation of Atoms
To change energy levels, an electron must either absorb or emit a photon that has the same amount of energy as the difference between the energy levels
E = h = hc/ --- A larger energy difference means a higher frequency.
Different jumps in energy levels means different frequencies of light absorbed, i.e. different colors

54. Kirchhoff’s Laws of Radiation
A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus produce a continuous spectrum. 54

55. Kirchhoff’s Laws of Radiation
2. A low-density gas excited to emit light will do so at specific wavelengths and thus produce an emission spectrum.
Light excites electrons in atoms to higher energy states
Transition back to lower states emits light at specific frequencies 55

56. Light excites electrons in atoms to higher energy states
Kirchhoff’s Laws of Radiation
3. If light comprising a continuous spectrum passes through a cool, low- density gas, the result will be an absorption spectrum.
Light excites electrons in atoms to higher energy states
Frequencies corresponding to the transition energies are absorbed from the continuous spectrum. 56

57. Sources of spectral lines
Emission nebula
Reflection nebula

59. The Spectrum of a star (the Sun)
There are similar absorption lines in the other regions of the electromagnetic spectrum. Each line exactly corresponds to chemical elements in the stars.

60. The spectrum of a star’s light is approximately a black body spectrum.
The spectrum of a star: nearly a Black Body
The light from a star is usually concentrated in a rather narrow range of wavelengths.
The spectrum of a star’s light is approximately a black body spectrum.
In fact, the spectrum of a star at the photosphere, before the light passes through the atmosphere of the star, is a nearly PERFECT black body one 60

61. Again, remember the two Laws of Black Body Radiation. I
1. The hotter an object is, the more energy it emits:
L = 4 R2T4
More area, more energy
where
L = Energy =
= Energy given off in the form of radiation, per unit time [J/s];
 = Stefan-Boltzmann constant 61

62. (where TK is the temperature in Kelvin)
Again, remember the two Laws of Black Body Radiation. II
2. The peak of the black body spectrum shifts towards shorter wavelengths when the temperature increases.
 Wien’s law:
max ≈ 2,900,000 nm / TK
(where TK is the temperature in Kelvin) 62

63. The spectra of stars also contain characteristic absorption lines.
Stellar Spectra
The spectra of stars also contain characteristic absorption lines.
With what we have learned about atomic structure, we can now understand how those lines are formed. 63

64. The Spectra of Stars
The inner, dense layers of a star do produce a continuous (blackbody) spectrum.
Cooler surface layers absorb light at specific frequencies.
The atmosphere also absorbs light at other specific frequencies
=> Spectra of stars are B.B.absorption spectra. 64

65. By far the most abundant elements in the Universe
Analyzing Absorption Spectra
Each element produces a specific set of absorption (and emission) lines.
Comparing the relative strengths of these sets of lines, we can study the composition of gases.
By far the most abundant elements in the Universe 65

66. Lines of Hydrogen
Most prominent lines in many astronomical objects: Balmer lines of hydrogen 66

67. The only hydrogen lines in the visible wavelength range
The Balmer Absorption Lines
n = 1
n = 4
Transitions from 2nd to higher levels of hydrogen
n = 5
n = 3
n = 2 H H H
The only hydrogen lines in the visible wavelength range
2nd to 3rd level = H (Balmer alpha line)
2nd to 4th level = H (Balmer beta line) 67

68. Emission nebula, dominated by the red H line
Observations of the H-Alpha Line
Emission nebula, dominated by the red H line 68

69. Absorption Spectrum Dominated by Balmer Lines
Modern spectra are usually recorded digitally and represented as plots of intensity vs. wavelength 69

70. The Balmer Thermometer
Balmer line strength is sensitive to temperature:
Most hydrogen atoms are ionized => weak Balmer lines
Almost all hydrogen atoms in the ground state (electrons in the n = 1 orbit) => few transitions from n = 2 => weak Balmer lines 70

71. Comparing line strengths, we can measure a star’s surface temperature!
Measuring the Temperatures of Stars
Comparing line strengths, we can measure a star’s surface temperature! 71

72. Spectral Classification of Stars
Different types of stars show different characteristic sets of absorption lines.
Temperature 72

73. Mnemonics to remember the spectral sequence:
Spectral Classification of Stars
Mnemonics to remember the spectral sequence: Oh
Only Be
Boy,
Bad A An
Astronomer s
Fine F
Forget
Girl/Guy
Grade
Generally
Kiss
Kills
Known Me
Mnemonics 73

74. Stellar Spectra O B A F
Surface temperature G K M 74

75. The Composition of Stars
From the relative strength of absorption lines (carefully accounting for their temperature dependence), one can infer the composition of stars. 75