1. Light and Matter

2. Light in Everyday Life Our goals for learning:
How do we experience light?
How do light and matter interact?
The warmth of sunlight tells us that light is a form of energy.
Energy is measured in joules. We can measure the flow of energy in light in units of watts: 1 watt = 1 joule/s

3. Interactions of Light White light is made up of many different colors
4 process:
Emission
Absorption
Transmission
Transparent objects transmit (allow to pass) light
Opaque objects block (absorb) light
Reflection or Scattering
Newton showed that white light is composed of all the colors of the rainbow.
White light is made up of many different colors

4. Reflection and Scattering
Mirror reflects light in a particular direction
Movie screen scatters light in all directions

5. Interactions of Light with Matter
Interactions between light and matter determine the appearance of everything around us. Objects with color (e.g. a red rose) appear that color because they absorb all the other colors and reflect (or scatter) that one.

6. What is light?
Light is a form of energy that can act either like a wave or like a particle (with energy and momentum) depending on its interaction with matter
A light wave is a vibration of electric and magnetic fields – an electromagnetic wave
Particles of light are bundles (quanta) of energy called photons

7. Waves
A wave is a pattern of motion that can carry energy without carrying matter along with it
Leave bobs up and down

8. Properties of Waves Wavelength is the distance between two wave peaks
Eyes are sensitive to wavelength; we sense wavelength differences as “color”.
Wavelength is the distance between two wave peaks
Frequency is the number of times per second that a wave vibrates up and down (cycles per second or hertz)
wave speed = wave length x frequency
Mathematically: c = f
: the Greek letter lambda

9. Particles of Light Particles of light are called photons
Each photon has a wavelength and a frequency associated with it
The Energy of a photon depends on its frequency, E = hf
h is a constant of nature called Planck’s constant

10. Wavelength, Frequency, and Energy
l x f = c
= wavelength, f = frequency
Visible Light: = 0.5 x 10-6 m, f = 6 x 1014 s-1 (Hz)
c = 3.00 x 108 m/s = speed of light
E = h x f = photon energy
h = x joule x s = Planck’s constant
One photon of visible light carries ~4 x joules of energy
A 100W bulb emits 2.5 x 1020 photons every second!

12. What have we learned? What is light?
Light can behave like either a wave or a particle
A light wave is a vibration of electric and magnetic fields
Light waves have a wavelength and a frequency
Photons are particles of light.
What is the electromagnetic spectrum?
Human eyes cannot see most forms of light.
The entire range of wavelengths of light is known as the electromagnetic spectrum.

13. Properties of Matter Our goals for learning:
What is the structure of matter?
What are the phases of matter
How is energy stored in atoms?
We need to know this in order to understand the end phase of a star’s life & what’s happening inside a white dwarf or neutron star.

14. What is the structure of matter?
Everything is made of atoms
Electrons have negative charge and are almost 2000x less massive than protons
Electron
Cloud
10-15 m
Nucleus
Atom
Use this figure to define the nucleus; protons, neutrons, electrons; scale of atom and “electron cloud.”
Neutron:no charge
Proton: positive charge
Volume of atom = 1,000 trillion times that of nucleus

15. Atomic Terminology Atomic Number = # of protons in nucleus
Atomic Mass Number = # of protons + neutrons
Molecules: consist of two or more atoms (H2O, CO2)

16. Atomic Terminology
Isotope: same # of protons but different # of neutrons. Example: (4He, 3He)

17. What are the phases of matter?
Solid (ice)
Liquid (water)
Gas (water vapor)
Plasma (ionized gas)
Phases of same material behave differently because of differences in chemical bonds. By chemical bonds we mean the electric forces between atoms.

18. Phase Changes
Read from bottom to top
Ionization: Stripping of electrons, changing atoms into plasma
Dissociation: Breaking of molecules into atoms
Evaporation: Breaking of flexible chemical bonds, changing liquid into gas
Melting: Breaking of rigid chemical bonds, changing solid into liquid

19. What have we learned? What is the structure of matter?
Matter is made of atoms, which consist of a nucleus of protons and neutrons surrounded by a cloud of electrons
What are the phases of matter?
Adding heat to a substance changes its phase by breaking chemical bonds.
As temperature rises, a substance transforms from a solid to a liquid to a gas, then the molecules can dissociate into atoms
Stripping of electrons from atoms (ionization) turns the substance into a plasma

20. Learning from Light Our goals for learning:
What are the three basic types of spectra?
How does light tell us what things are made of?
How does light tell us the temperatures of planets and stars?
How do we interpret an actual spectrum?

21. What are the three basic types of spectra?
Continuous Spectrum
Emission Line Spectrum
Absorption Line Spectrum
Spectra of astrophysical objects are usually combinations of these three basic types. We can take a picture of a spectrum (lower bar) or we can plot a graph of intensity versus wavelength (upper).

22. Continuous Spectrum
Slit in screen
The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption

23. Emission Line Spectrum
Each colored “line” is an image of the entrance slit.
A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines

24. Absorption Line Spectrum
A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum

25. Chemical Fingerprints
Electrons in atoms can only occupy certain energy states or levels
The lowest energy state (level 1) is the Ground State
Downward transitions between energy states produce a unique pattern of emission lines (E = hf = hc/)

26. Chemical Fingerprints
Because those atoms can absorb photons with those exact same energies, upward transitions produce a pattern of absorption lines at the same wavelengths

27. Chemical Fingerprints
Each type of atom has a unique spectral fingerprint

28. Chemical Fingerprints
Observing the fingerprints in a spectrum tells us which kinds of atoms are present

29. Energy Levels of Molecules
Molecules have additional energy levels because they can vibrate and rotate
The “spring” just represents the electrical bond between the atoms of the molecule

30. Energy Levels of Molecules
The large numbers of vibrational and rotational energy levels can make the spectra of molecules very complicated
Many of the energy transitions due to vibration and rotation of molecules occur in the infrared part of the spectrum

31. Thermal Radiation
Nearly all large or dense objects emit thermal radiation, including stars, planets, you…
Collisions between atoms in a hot object causes electrons to jump to higher energy levels for a while and then drop down again to emit light. As a result, the photons produced are intimately linked with the temperature (average kinetic energy) in the collisions. Radiation produced this way is called thermal.
An object’s thermal radiation spectrum depends on only one property: its temperature

32. Properties of Thermal Radiation
Hotter objects emit more light at all frequencies per unit area. Power per sq. meter = σT4 (Stefan’s Law)
Hotter objects emit photons with a higher average energy. maxT ~ (for  in m)
(Wien’s Law)
Remind students that the intensity is per area; larger objects can emit more total light even if they are cooler.
Larger objects can emit more total light even if they are cooler. For a sphere (star), luminosity is L = 4πR2σT4

33. Thought Question Why don’t we glow in the dark?
People do not emit any kind of light.
People only emit light that is invisible to our eyes.
People are too small to emit enough light for us to see.
People do not contain enough radioactive material.

34. Thought Question Why don’t we glow in the dark?
People do not emit any kind of light.
People only emit light that is invisible to our eyes. We glow in the infrared.
People are too small to emit enough light for us to see.
People do not contain enough radioactive material.

35. What have we learned? What are the three basic type of spectra?
Continuous spectrum, emission line spectrum, absorption line spectrum
How does light tell us what things are made of?
Each atom has a unique fingerprint.
We can determine which atoms something is made of by looking for their fingerprints in the spectrum.

36. What have we learned?
How does light tell us the temperatures of planets and stars?
Nearly all large or dense objects emit a continuous spectrum that depends on temperature.
The spectrum of that thermal radiation tells us the object’s temperature.
How do we interpret an actual spectrum?
By carefully studying the features in a spectrum, we can learn a great deal about the object that created it.

37. The Doppler Effect Our goals for learning:
How does light tell us the speed of a distant object?
How does light tell us the rotation rate of an object?

38. How does light tell us the speed of a distant object?
The Doppler Effect
This figure from the book can give an introduction to the Doppler effect.
Waves are compressed in the direction of motion  wavelength is decreased frequency is higher
Same thing happens for light.

39. Measuring the Shift Stationary Moving Away Away Faster Moving Toward
Toward Faster
We generally measure the Doppler Effect from shifts in the wavelengths of spectral lines
The fractional shift is: ( - 0)/0 where 0 is the undisturbed wavelength; this number is equal to the speed of the object relative to that of light (V/c)

40. Doppler shift tells us ONLY about the part of an object’s motion toward or away from us:
Pure radial motion – maximum Doppler shift
Transverse motion – no Doppler shift
Part radial, part transverse – Doppler shift gives Vr = Vcosθ (less than V) Vr θ

41. Thought Question I measure a spectral line in the lab at 500. 7 nm
Thought Question I measure a spectral line in the lab at nm. The same line in a star has wavelength nm. What can I say about this star?
It is moving away from me.
It is moving toward me.
It has unusually long spectral lines.

42. It is moving away from me. This is a REDSHIFT It is moving toward me.
Thought Question I measure a spectral line in the lab at nm. The same line in a star has wavelength nm. What can I say about this star?
It is moving away from me. This is a REDSHIFT
It is moving toward me.
It has unusually long spectral lines.
And redshift = (502.8 – 500.7)/500.7 =
Therefore the radial component of velocity = x c
= 1,258 km/s

43. Measuring Redshift

44. How does light tell us the rotation rate of an object?
Spectrum of a Rotating Object
Slower
This figure from the book can give an introduction to the Doppler effect.
Faster
Spectral lines are wider when an object rotates faster

45. What have we learned?
How does light tell us the speed of a distant object?
The Doppler effect tells us how fast an object is moving toward or away from us.
Blueshift:objects moving toward us
Redshift: objects moving away from us
How does light tell us the rotation rate of an object?
The width of an object’s spectral lines can tell us how fast it is rotating

46. Telescopes: Portals of Discovery

47. A Selection of Major Telescopes
Mauna Kea, Hawaii
4 x 8m
Chile
2 x 10m
X-ray telescope in space
Hubble: 2.5m in space
New Mexico
VLA:27 radio telescopes

48. Eyes and Cameras: Everyday Light Sensors
Our goals for learning:
How does your eye form an image?
How do we record images?

49. How does your eye form an image?
The lens bends (refracts) light rays and brings to a focus on retina.

50. Refraction
Refraction is the bending of light when it passes from one substance into another
Your eye uses refraction to focus light

51. Focusing Light
Refraction can cause parallel light rays to converge to a focus

52. Image Formation
The focal plane is where light from different directions comes into focus
The image behind a single (convex) lens is actually upside-down!

53. Focusing Light
Digital cameras detect light with charge-coupled devices (CCDs)
A camera focuses light like an eye and captures the image with a “detector”
The electronic detectors in digital cameras are similar to those used in modern telescopes

54. What have we learned? How does your eye form an image?
It uses refraction to bend parallel light rays so that they form an image.
The image is in focus if the focal plane is at the retina.
How do we record images?
Cameras focus light like your eye and record the image with a detector.
The detectors (CCDs or charge-coupled devices) in digital cameras are like those used on modern telescopes; these devices convert photons into electrons and then into an electronic image that gets stored in a computer

55. Telescopes: Giant Eyes
Our goals for learning:
What are the two most important properties of a telescope?
What are the two basic designs of telescopes?
What do astronomers do with telescopes?

56. What are the two most important properties of a telescope?
Light-collecting area: Telescopes with a larger collecting area can gather a greater amount of light in a shorter time.
Angular resolution: Telescopes that are larger are capable of taking images with greater detail.

57. Light Collecting Area
A telescope’s diameter tells us its light-collecting area: Area = π(diameter/2)2
The largest telescopes currently in use have a diameter of about 10 meters: these are the twin 10m telescopes of the W. M. Keck Observatory which are owned and operated by the California Association for Research in Astronomy (UC and Caltech).

58. Bigger is better
Tool from the Telescopes tutorial.

59. Thought Question How does the collecting area of a 10-meter telescope compare with that of a 2-meter telescope?
It’s 5 times greater.
It’s 10 times greater.
It’s 25 times greater.

60. Thought Question How does the collecting area of a 10-meter telescope compare with that of a 2-meter telescope?
It’s 5 times greater.
It’s 10 times greater.
It’s 25 times greater. Fives times larger in diameter means 25 times more area.

61. Angular Resolution
The minimum angular separation that the telescope can distinguish.
At a great distance the pair of lights will look like one rather than two; the light will still be visible but we will not be able to distinguish that it is from two sources.
Emphasize that at a great distance the lights would look like one light rather than two --- but we could still see them. (Students sometimes think that below a particular angular resolution we see nothing at all.)

62. Angular Resolution
Ultimate limit to resolution comes from interference of light waves within a telescope.
Larger telescopes are capable of greater resolution because there’s less interference

63. Angular Resolution
The rings in this image of a star come from interference of light wave.
This limit on angular resolution is known as the diffraction limit
Angular resolution scales as /D
Close-up of a star from the Hubble
Space Telescope
 = wavelength; D = telescope diameter

64. What are the two basic designs of telescopes?
Refracting telescope: Focuses light with lenses
Reflecting telescope: Focuses light with mirrors

65. Refracting Telescope
Refracting telescopes need to be very long, with large, heavy lenses
Largest lens is ~1m diameter

66. Reflecting Telescope
Reflecting telescopes can have much greater diameters; invented by Isaac Newton
Most modern telescopes are reflectors

67. Designs for Reflecting Telescopes

68. Mirrors in Reflecting Telescopes
Twin Keck telescopes on
Mauna Kea in Hawaii
Segmented 10-meter mirror of a Keck telescope
Hard to make really big mirrors by grinding glass. The Keck telescopes have 36 hexagonal segments computer-controlled to act like one large curved mirror.

70. What do astronomers do with telescopes?
Imaging: Taking pictures of the sky
Spectroscopy: Breaking light into spectra
Timing: Measuring how light output varies with time

71. Imaging
Astronomical detectors generally record only one color of light at a time
Several images must be combined to make full-color pictures

72. Imaging
Astronomical detectors can record forms of light our eyes can’t see
False Color is sometimes used to represent different energies of non-visible light

73. Spectroscopy
A spectrograph separates the different wavelengths of light before they hit the detector
Diffraction
grating breaks
light into
spectrum
Light from
only one star
enters
Detector
records
spectrum

74. Spectroscopy
Graphing relative brightness of light at each wavelength shows the details in a spectrum

75. Timing
A light curve represents a series of brightness measurements made over a period of time

76. What have we learned?
What are the two most important properties of a telescope?
Collecting area determines how much light a telescope can gather
Angular resolution is the minimum angular separation a telescope can distinguish
What are the two basic designs of telescopes?
Refracting telescopes focus light with lenses
Reflecting telescopes focus light with mirrors
The vast majority of professional telescopes are reflectors

77. What have we learned? What do astronomers do with telescopes? Imaging
Spectroscopy
Timing

78. Telescopes and the Atmosphere
Our goals for learning:
How does Earth’s atmosphere affect ground-based observations?
Why do we put telescopes into space?

79. How does Earth’s atmosphere affect ground-based observations?
The best ground-based sites for astronomical observing are
Calm (not too windy)
High (less atmosphere to see through)
Dark (far from city lights)
Dry (few cloudy nights)

80. Light Pollution
Scattering of human-made light in the atmosphere is a growing problem for astronomy

81. Twinkling and Turbulence
Star viewed with ground-based telescope
Same star viewed with Hubble Space Telescope
Turbulent air flow in Earth’s atmosphere distorts our view, causing stars to twinkle and images to blur

82. Adaptive Optics A new technology that corrects for atmospheric turbulence
Without adaptive optics
With adaptive optics
How is it done? Rapidly changing the shape of a telescope’s mirror can compensate for some of the effects of turbulence

83. Another example of adaptive optics.

84. Calm, High, Dark, Dry
The best observing sites are atop remote mountains
This is where many astronomers work.
Summit of Mauna Kea, Hawaii

85. Why do we put telescopes into space?

86. Transmission in Atmosphere
Only radio and visible electromagnetic waves pass easily through Earth’s atmosphere
We need telescopes in space to observe other forms

87. What have learned?
How does Earth’s atmosphere affect ground-based observations?
Telescope sites are chosen to minimize the problems of light pollution, atmospheric turbulence, and bad weather.
Why do we put telescopes into space?
Forms of light other than radio and visible do not pass through Earth’s atmosphere.
Also, much sharper images are possible because there is no turbulence.

88. Telescopes and Technology
Our goals for learning:
How can we observe non-visible light?
How can multiple telescopes work together?

89. How can we observe non-visible light?
A standard satellite dish is essentially a telescope for observing radio waves

90. Radio Telescopes
A radio telescope is like a giant mirror that reflects radio waves to a focus
Arecibo: 300m

91. Infrared Telescopes
SOFIA
Spitzer
Infrared (and ultraviolet-light) telescopes operate like visible-light telescopes but need to be above atmosphere to see all IR (and UV wavelengths); examples of a UV telescope – Hubble, GALEX

93. X-Ray Telescopes X-ray telescopes also need to be above the atmosphere
Chandra
X-rays are harder to focus …

94. X-Ray Telescopes Focusing of X-rays requires special mirrors
Mirrors are arranged to focus X-ray photons through grazing bounces off the surface

95. Gamma Ray Telescopes Gamma ray telescopes also need to be in space
Focusing gamma rays is extremely difficult
Compton Observatory
Current Missions: SWIFT, FERMI

96. How can multiple telescopes work together?

97. Interferometry
Interferometry is a technique for linking two or more telescopes so that they have the angular resolution of a single large one

98. Interferometry Easiest to do with radio telescopes
Now becoming possible with infrared and visible-light telescopes also
Very Large Array (VLA), Socorro, New Mexico

99. Concept for a 30-meter Giant Segmented Mirror Telescope