1. 5. The Nature of Light Light travels in vacuum at 3.0 . 108 m/s
Light is one form of electromagnetic radiation
Continuous radiation: Based on temperature
Wien’s Law & the Stefan-Boltzmann Law
Light has both wave & particle properties
Each element has unique spectral lines
Atoms: A nucleus surrounded by electrons
Spectral lines: Electrons change energy levels
Spectral lines shift wavelength due to motion

2. Does Light Travel Infinitely Fast?
Some ancient common experiences
Lightning & thunder
At minimum, light travels faster than easily measured
At maximum, light might travel infinitely fast
Galileo’s experiments
Human reflexes are much too slow
Human pulse is much too long
Olaus Rømer 1676
Inconsistencies in occultations of Jupiter’s moons
Earlier than expected with Jupiter closer than average
Later than expected with Jupiter farther than average

3. EMR Travels At Finite Speed
Occultation

4. Light Moves in Vacuum 3.0 . 108 m/s
Light travels at constant speed in vacuum
Recognized by Einstein as highest possible speed
Independent of the speed of any observer
That speed is…c…and is… “celeritas”
c = km/s
c = m/s
c = cm/s
Light travels different speeds in different media
Air slows light a little Low density
Light bends/refracts a little as it enters the atmosphere
Glass slows light a lot High density
Light bends/refracts a lot as it enters a telescope lens

5. “Light” is Electromagnetic Radiation
“Light” is one form of electromagnetic radiation
Electric & magnetic components are sine waves
Electric & magnetic components identical wavelengths
Electric & magnetic components perfectly synchronized
Various regions electromagnetic radiation
R Radio Longest λ’s Low energies
I Infrared
V Visible “Light” Medium energies
U Ultraviolet
X X-ray
G Gamma-ray Shortest λ’s High energies

6. EMR: Electric & Magnetic Waves
Wave properties
Electric vector vibrates in a sine wave form
vibrates in a single plane
Magnetic vector vibrates in a sine wave form
vibrates perpendicular to e– vector
vibrates synchronized w/e– vector

7. Refraction of Sunlight By a Prism
The “Celebrated Phenomenon of Colours”
Red light is
refracted least
Blue light is
refracted most

8. Prisms Do Not Add Color to Light
Newton’s prism experiments
Isolate one color from sunlight using one prism
Pass that color through a second prism
No color is added

9. The Electromagnetic Spectrum

10. Emission & Absorption Spectra
Emission spectra Bright = Hot
Looking directly at a hot high-density object
Continuous  Hot high-density objects
Hot stars with no intervening interstellar gas clouds
Bright-line  Hot low-density objects
Hot interstellar gas clouds between any star & the Earth
Absorption spectra Dark = Cold
Not looking directly at a hot high-density object
Dark-line  Cool low-density objects
Cool interstellar gas clouds

11. Continuous and Line Spectra
Absorption from a cool
low density object
Emission from a hot Emission from a hot
high density object low density object = +

12. The Blackbody Concept Blackbody: An ideal concept Wien’s Law
Absorbs 100% of all wavelengths of incident EMR
All X-rays, visible light, radio waves…
Experience shows that this is impossible
Emits all absorbed energy as blackbody radiation
Radiation based exclusively on Kelvin temperature
Experience shows that this actually happens
Wien’s Law
Wavelength at which the most energy is produced
Stefan-Boltzmann Law
Total energy is proportional to T4

13. Blackbody Curve: The Ideal
“White” stars
Our Sun
“Red” stars

14. Blackbody Curve: The Sun

15. Wien’s Law Blackbody radiation curves have one peak
This wavelength emits the most energy
This wavelength depends on Kelvin temperature
lmax = Wavelength of maximum emission (meters)
T = Temperature (kelvins)
max is inversely proportional to Kelvin temp.
Higher temperature  Shorter wavelength

16. The Stefan-Boltzmann Law
Blackbody radiation curves show energy flux
This energy flux depends on Kelvin temperature
F = Energy flux (joules . m–2 . sec–1 )
s = Constant = –8 W . m–2 . K–4
TK = Temperature (kelvins)
Energy is directly proportional to TK4
Raising TK by a factor of 10 raises energy by 10,000

17. The Wave-Particle Nature of EMR
EMR behavior depends on the experiment
Wave experiment: EMR behaves like a wave
Young’s double-slit experiment
Particle experiment: EMR behaves like a particle
EMR as photons
A quantum amount of EMR energy
Energy = Planck’s Constant . Frequency
The photoelectric effect
Electron emission requires some minimum energy
Possible only if photons actually exist

18. Each Element Has a Unique Spectrum
Every material has a unique spectral signature
Unique set of spectral lines
When hot, the spectral lines are bright
When cool, the spectral lines are dark
Each spectral line has a unique  Spectroscopy
Each spectral line emits a unique amount of energy
Kirchhoff’s Laws
Hot opaque objects: Continuous spectra
Classical blackbody radiation
Hot transparent objects: Bright-line spectra
Hot interstellar gas clouds with no continuous background
Cool transparent objects: Dark-line spectra
Cool interstellar gas clouds with a continuous background

19. The Periodic Table of the Elements

20. Spectra: The Hydrogen Family

21. Spectra: The Helium Family

22. Spectra: The Beryllium Family

23. Spectra: The Boron Family

24. Spectra: The Carbon Family

25. Spectra: The Nitrogen Family

26. Spectra: The Oxygen Family

27. Spectra: The Fluorine Family

28. The Bohr Model of the Atom
A central nucleus
One or more protons Atomic number
Determines the chemical properties (elements)
Zero or more neutrons Mass number
Determines the nuclear properties (isotopes)
Electron orbitals surround the nucleus
Neutral atoms: Number of p+ = Number of e–
Ionized atoms: Number of p+ ≠ Number of e–
Cations: One or more e– lost Net positive charge
Anions: One or more e– gained Net negative charge

29. Bohr Model of the Hydrogen Atom
Electron orbitals are not to scale

30. Hydrogen Electron Transitions

31. Electrons Jump Energy Levels
Electrons jumping energy levels produce lines
Hydrogen atom is the simplest of all
Lyman series: Ultraviolet spectrum
Balmer series: Visible spectrum
Paschen series: Infrared spectrum
All other atoms & elements are more complicated
More considerations about spectral lines
Each line has a different amount of energy
Energy = Planck’s constant . Frequency
Each line has a different probability of jumping
More jumps  More energy emitted  Brighter lines

32. Spectra: Hydrogen Energy Levels

33. The Doppler Effect Effect Wavelength shift due to relative motion
Source & viewer moving closer Blue shift
Spectral lines shifted toward blue end of the spectrum
The spectral lines do not actually appear blue ! ! !
Source & viewer moving farther Red shift
Spectral lines shifted toward red end of the spectrum
The spectral lines do not actually appear red ! ! !
Cause Relative motion of source & observer
Source & viewer moving closer
Waves compressed  Shorter wavelength  Blue shift
Source & viewer moving farther
Waves stretched  Longer wavelength  Red shift

34. Doppler Shift: Stretching Waves
Compressed wavelengths Stretched wavelengths
Higher frequencies Lower frequencies
Shift toward blue Shift toward red

35. Important Concepts Light in vacuum at constant speed
m . sec–2
Light in other media moves slower
Related generally to media density
Light is one form of EMR
Gamma rays
X-rays
Ultraviolet
Visible
Infrared
Microwave / Radio
Emission & absorption spectra
Continuous Hot high density
Bright line Hot low density
Dark line Cool low density
Blackbody concept
Absorbs 100% of all wavelengths
Emits % at specific wavelengths
Wien’s Law
Wavelength of maximum energy
Stefan-Boltzmann Law
Total energy produced
Wave-particle duality of all EMR
Behavior depends on experiment
Photoelectric effect
Unique sets of spectral lines
Kirchhoff’s three laws
Bohr’s mode of hydrogen
Nucleus with orbitals
Neutral & ionized atoms
Electron energy jumps produce lines
Doppler effect
Relative convergence: Blue shift
Relative divergence: Red shift