1. Light is an ELECTROMAGNETIC WAVE
THE NATURE OF LIGHT
Light is an ELECTROMAGNETIC WAVE
Light is also a PARTICLE: the PHOTON
This nominal contradiction is an example of
COMPLEMENTARITY or DUALITY in QUANTUM MECHANICS Depending on circumstances it is preferable to use one or the other point of view for light, electrons, protons, atoms: anything which is too small to be described by normal physics and directly experience requires using quantum mechanics

2. First, the Wave Nature Light is a TRANSVERSE ELECTROMAGNETIC WAVE.
Electric (E) fields oscillate perpendicular to
Magnetic (B) fields and the ENERGY FLOWS PERPENDICULAR to both fields.
Other transverse waves: WATER waves, SECONDARY (shear) SEISMIC waves.
LONGITUDINAL WAVES have ENERGY FLOWS PARALLEL to oscillations: SOUND, PRIMARY (compressional) SEISMIC waves.

3. Transverse Electromagnetic (EM) Wave

4. Electric and Magnetic Fields
Charged particles (protons or ions +, electrons -) attract or repel each other.
Electric fields accelerate charged particles along the lines.
Charged particles orbit around magnetic field lines.

5. Characteristics of All Waves
Frequency (f or  [nu]): oscillations per sec (Hz)
Speed (v): depends on medium, sometimes  (cm/s or m/s)
Wavelength ( [lambda]): distance between crests (cm)
Amplitude (A): strength of oscillation
Anatomy of a Wave Applet
Start here on Mon 8/28

6. Key Relations for ALL Waves
Speed = Wavelength x Frequency v = f
Or f = v/  or  = v/f
Power Amplitude P  A2
Surface Waves in a Pond

7. Wavelength and Frequency for Light
wavelength x frequency = speed of light = constant

8. For EM Waves ONLY v = c = 2.9979x108 m s-1 = 3.00x1010 cm s-1
= 3.00x105 km s-1 = 186,000 miles/s
The speed of light does not depend upon direction or frequency in a vacuum and doesn’t need a medium It is a CONSTANT of NATURE.
In matter, v < c and the same frequency has a shorter wavelength than in vacuum The INDEX OF REFRACTION, n = c/v >= In air, n = ; in normal glass, n  1.5
Start here on 1/14

9. Special Topic: Polarized Light
EM WAVES CAN ALSO BE POLARIZED:
E field in a particular plane; B field in one perpendicular plane  LINEAR POLARIZATION: this is the only kind of polarization we'll worry about.
Reflection can change the polarization of light
Polarized sunglasses block light that reflects off of horizontal surfaces

10. The Seven Bands of the EM Spectrum
 Microwave or millimeter between Radio and IR

11. Atmospheric Transmission
Radio  > 1 cm -- The longest waves or lowest frequencies.
Penetrates atmosphere if <15 m
So AM reflects off ionosphere while FM penetrates
Millimeter or microwave: 1 cm >  > cm -- partially penetrates atm; molecules absorb.
Infrared (IR) cm >  > 7.2x10-5 cm = 720 nm
CO2 , H2O etc absorb most but some s penetrate

13. Visible Wavelengths
VISIBLE (OPTICAL): nm = 7200 Å >  > 380 nm = 3800 Å, x Hz < f < 7.9 x Hz.
Penetrates atmosphere (shorter scatter more)
Visible spectrum: RED (longest wavelength), ORANGE, YELLOW, GREEN, BLUE, VIOLET (shortest wavelength -- highest frequency)
Our eyes evolved to see this light, since the Sun produces most of its radiation in this band, and since nearly all of this radiation gets through the atmosphere.
Visible Light Applet

14. Colors of Light White light is made up of many different colors
Newton showed that white light is composed of all the colors of the rainbow.
White light is made up of many different colors

15. Shortest Wavelengths
ULTRAVIOLET (UV): 380 nm >  > 300Å = 30nm Mostly absorbed in atmosphere: ozone (O3) Good thing, since UV radiation causes skin cancer.
X-RAY: 300 Å >  > Å = 0.01nm, Absorbed in atmosphere: by any atom (N, O) A good thing too: X-rays can penetrate the body and cause cancer in many organs.
GAMMA-RAY (-ray):  < 0.1 Å =0.01 nm, The most energetic form of EM radiation Absorbed high in atmosphere: by any atomic nucleus A VERY good thing: gamma-rays quickly cause severe burns and cancer.

16. Blue light is (compared to red light),
Shorter wavelength
Longer wavelength
Higher energy photons
1 and 3
None of the above

17. Blue light is (compared to red light),
Shorter wavelength
Longer wavelength
Higher energy photons
1 and 3
None of the above

18. We can’t see infrared, but we can perceive it as:
Heat
Radar
Sound AM FM

19. We can’t see infrared, but we can perceive it as:
Heat
Radar
Sound AM FM

20. How are Electromagnetic Waves Made?
Most come from ATOMIC, MOLECULAR or NUCLEAR TRANSITIONS I.e., electrons or protons changing quantum states.
BUT FUNDAMENTALLY, EM RADIATION IS PRODUCED BY AN ACCELERATED CHARGED PARTICLE.
Since ELECTRONS have the LOWEST MASSES they are MOST EASILY ACCELERATED, therefore, electrons produce most EM waves.

21. Examples of EM Wave Generation
Radio - TV - Cell Phone transmission towers. Electrons oscillate up & down
Synchrotron Radiation produced by electrons spiraling around magnetic field lines, when moving at nearly speed of light.
The circular part of the motion is ACCELERATED and produces the radiation. Synchrotron radiation is strongly POLARIZED; most EM radiation is basically UNPOLARIZED
Start here on Wed 8/30

22. How do Waves Interact with Matter?
EMIT (light is sent out when a bulb is turned on)
REFLECT (angle of incidence = angle of reflection) or Scatter (spread out reflection)
TRANSMIT (low opacity)
ABSORB (high opacity)
REFRACT (bend towards normal when entering a medium with a slower propagation speed)
INTERFERE (only a WAVE can do this) Either: CONSTRUCTIVE (waves add when in phase) DESTRUCTIVE (waves cancel when out of phase)
DIFFRACT (only a WAVE can do this) Waves spread out when passing through a hole or slit. This is important only if the size of the hole or slit is comparable to the wavelength.
Start here on Thurs 8/31

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

24. Interactions of Light with Matter
Interactions between light and matter determine the appearance of everything around us: objects reflect some wavelengths, absorb others and emit others.

25. When light approaches matter, it can
Be absorbed by the atoms in the matter
Go through the matter, and be transmitted
Bounce off the matter, and be reflected
Any of the above
Only 2 or 3

26. When light approaches matter, it can
Be absorbed by the atoms in the matter
Go through the matter, and be transmitted
Bounce off the matter, and be reflected
Any of the above
Only 2 or 3

27. Thought Question Why is a rose red?
The rose absorbs red light.
The rose transmits red light.
The rose emits red light.
The rose reflects red light.

28. Thought Question Why is a rose red?
The rose absorbs red light.
The rose transmits red light.
The rose emits red light.
The rose reflects red light.

29. Interference and Diffraction

30. Light as Particles ELECTROMAGNETIC ENERGY IS CARRIED BY PHOTONS:
A PHOTON is a SINGLE QUANTUM OF LIGHT The energy of one photon of a particular frequency is:
E = hf = h c /  h = x Joule sec = x erg sec is PLANCK's CONSTANT.
Along with c, the speed of light; e, the charge on an electron (or proton) and G (Newton's constant of gravity), h is one of the
FUNDAMENTAL CONSTANTS of NATURE.
Start here on 9/15/09

31. Thought Question The higher the photon energy…
the longer its wavelength.
the shorter its wavelength.
energy is independent of wavelength.

32. Thought Question The higher the photon energy…
the longer its wavelength.
the shorter its wavelength.
energy is independent of wavelength.

33. Photons vs. Waves These PHOTONS can equally well explain REFLECTION,
REFRACTION,
TRANSMISSION and
ABSORPTION
as can the Wave picture,
BUT they can't explain
INTERFERENCE and
DIFFRACTION.

34. Photons vs. Waves, Round 2
On the other hand the WAVE picture can't explain:
The PHOTOELECTRIC EFFECT
(where metals emit electrons when light shines on them)
and SPECTRAL LINES
(where only specific wavelengths of light emerge from particular elements)
while the PARTICLE part of the duality in Quantum Mechanics CAN!
We’ll soon discuss each of these key aspects of light: the latter is at the core of modern astronomy.

35. How can light behave as both a wave and a particle?
It doesn’t really
It really is simultaneously both a wave and a particle
Light and small objects such as atoms behave in ways we never see in everyday objects, so we can’t describe them in everyday terms
This is what quantum mechanics describes
3 and 4

36. How can light behave as both a wave and a particle?
It doesn’t really
It really is simultaneously both a wave and a particle
Light and small objects such as atoms behave in ways we never see in everyday objects, so we can’t describe them in everyday terms
This is what quantum mechanics describes
3 and 4

37. Radiation, Temperature and Power
Crudely, hotter matter produces more highly accelerated charged particles, which therefore produces more powerful EM radiation.
Heat energy is proportional to temperature:
E = k T (where T is in Kelvins, 0 at ABSOLUTE ZERO).
So the thermal (heat) energy in atoms should be proportional to the photon energy: using math
h f  kT OR
  1/T

38. Temperature Scales Only the US has stuck with Fahrenheit temperatures;
The rest of the world normally uses Celcius, but
ENERGIES VANISH AT ABSOLUTE ZERO: THE NATURAL TEMPERATURE SCALE IS KELVINS.
The size of 1 degree C = 1 K and = 1.8 degrees F.
0 C = K (round it off)
The conversion formula is: F = (9/5)*C + 32
or C = (5/9)*(F - 32)

39. Wien’s Law Or, max=2,900,000/T (nm)
This is the PEAK WAVELENGTH for BLACKBODY (or Thermal, or Planckian) emission from a SOLID, a LIQUID or a DENSE GAS.
Ex: T = 5800K = 5.8x103K
=0.5x10-4cm=5x10-5cm
=500 nm = 5000Å
Wien's Law Applet

40. Thermal Spectra

41. Properties of Thermal Radiation
Hotter objects emit more light at all frequencies per unit area.
Hotter objects emit photons with a higher average energy.
Remind students that the intensity is per area; larger objects can emit more total light even if they are cooler.

42. Thought Question Which is hotter?
A blue star.
A red star.
A planet that emits only infrared light.

43. Thought Question Which is hotter?
A blue star.
A red star.
A planet that emits only infrared light.

44. Thought Question Why don’t we glow in the dark?
People do not emit any kind of light.
People essentially 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.

45. Thought Question Why don’t we glow in the dark?
People do not emit any kind of light.
People essentially 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.

46. The Photoelectric Effect
Electrons can be expelled from many materials if light shines upon them.
If the wavelength is TOO LONG (low frequency) nothing happens,
EVEN IF the INTENSITY of the light is HIGH.
Above a CRITICAL FREQUENCY the emitted electrons have a maximum energy (or velocity) that RISES with the FREQUENCY.
Ee = h f - h fcrit
Increasing the INTENSITY of light above the critical frequency increases only the number of ejected electrons, but NOT their energies.

47. Photoelectric Effect, Illustrated

48. Importance of Photoelectric Effect
Einstein pointed out that the wave theory could not explain this, while quanta of energy, with E = h f could.
The wave theory predicted that even red light, if intense enough, would eject electrons but this never happened.
The wave theory also said that as the blue light was made brighter, faster electrons would emerge:
instead only more of them came out,
but their maximum kinetic (motion) energy was a function ONLY of the light's FREQUENCY.

49. The Stefan-Boltzmann Law
Integrate (add up) a blackbody spectrum and find that the FLUX, or ENERGY/TIME/AREA is given by
F =  T where  = 5.67x10-8W m-2 K-4 =5.67x10-5erg s-1cm-2K-4
POWER = FLUX x AREA, or, for a sphere (of AREA = 4  R2) L = 4   R2 T4
For example, double T and raise L 16 times! Or raise T by 1% and L goes up about 4%.
Start here on 9/5/06

50. Peer Instruction Question: What happens to thermal radiation (a continuous spectrum) if you make the source hotter?
More energy comes out at all wavelengths
The peak of the spectrum-energy curve (the wavelength at which most energy is emitted) shifts redward
The peak of the spectrum-energy curve shifts blueward
1 and 2
1 and 3

51. What happens to thermal radiation (a continuous spectrum) if you make the source hotter?
More energy comes out at all wavelengths
The peak of the spectrum-energy curve (the wavelength at which most energy is emitted) shifts redward
The peak of the spectrum-energy curve shifts blueward
1 and 2
1 and 3

52. Luminosity, Temperature and Size
Since T can be measured by Wien's Law and L can be obtained from the star's or planet’s brightness and distance (we'll discuss later) this formula lets astronomers find the SIZES of planets and stars!
We’ll do that later; for now let’s compare luminosities:
Example: T1 = 500 K, R1 = 2,000 km;
T2 = 250 K, R2 = 4,000 km
L1/L2 = (2,000 km/4,000 km)2 (500 K/250 K)4
= (1/2)2 (2)4 = (1/4) 16 = 4 or L1/L2=4
In words, Planet 1 has 4 times the luminosity of Planet 2

53. WHAT IS THE STRUCTURE OF MATTER?
Electron
Cloud
Atom
Nucleus
Use this figure to define the nucleus; protons, neutrons, electrons; scale of atom and “electron cloud.”
Nucleus size only around 10-15m while electron clouds are roughly 10-10m = 0.1 nm = 1Å
As nearly all of the mass is in the nucleus, matter is mostly empty space!

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

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

56. What is found in the nucleus of atoms?
Protons with a + charge
Neutrons with no charge
Electrons with a – charge
All of the above
1 and 2

57. What is found in the nucleus of atoms?
Protons with a + charge
Neutrons with no charge
Electrons with a – charge
All of the above
1 and 2

58. How is the isotope 14C different from 12C?
It has more protons
It has more neutrons
It has more electrons
All of the above
None of the above

59. How is the isotope 14C different from 12C?
It has more protons
It has more neutrons
It has more electrons
All of the above
None of the above

60. What are the phases of matter?
Familiar phases:
Solid (ice)
Liquid (water)
Gas (water vapor)
Phases of same material behave differently because of differences in chemical bonds
Less familiar phase: ionized gas, where electrons are ripped off atoms.
But this is most of the normal matter in the Universe!

61. Phase Changes
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
Sublimation: from solid to gas

62. Review of the Nature of Matter
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