Properties of Light

The Speed of Light Galileo’s Experiment A B c = Round Trip Distance/Total Time

Ole Roemer (1675) Earth Jupiter

Speed of Light  From Roemer’s Experiment  c = Diameter of Orbit/Time Delay  Modern Value  c = 299, km/sec

Speed of Light  Absolute constant  Nothing travels faster

Relative Velocities  Our physical sense of velocities is relative  How we measure speed depends on how we are moving and how the object we are measuring in moving V rel = 0 v v vv V rel = 2v

Try it with Light Sun ½ c c Intuition says you get ½ c Einstein says c

Try it with Light Sun ½ c c Intuition says you get 1½ c Einstein says c

Nature of Light  Particle?  Newton  Wave?  Christian Huygens in Newton’s day  Thomas Young (1801)  Light can diffract - must be a wave  Transverse wave

Types of Waves  Transverse and Longitudinal Transverse and Longitudinal

Wavelength

Electromagnetic Spectrum Increasing Wavelength Visible Light

The Visible Spectrum 400nm 500nm 600nm 700nm Wavelength means COLOR

Frequency  Number of complete events occurring in a period of time  Waves/second  Examples  Second hand on a clock?  1 cycle/minute = 1/60 cycle/sec  US Presidential Electons  1 election/4 years = 1/4 election/year

Frequency and Period Period = 1 Frequency

Speed, Wavelength, Frequency c = f

Photon Energy E  f E = hf

Properties of Light  Speed (c) Nothing travels faster Absolute constant  Wavelength ( ) Gamma rays, X-rays, UV, Visible, IR, Radio In visible wavelength means color  Frequency (f) Number of complete waves per second  Energy of a Photon (E) E = hf

Dispersive Element Basic Spectrograph Collimating Lens Recording Device Slit Imaging Lens

Mercury near Horizon The atmosphere can act like a prism

Continuous Spectrum Source must be HOT and DENSE

Emisson (Bright Line) Spectrum Source must be HOT and TENUOUS

Absorption Spectrum

Types of Spectra Continuous Emission Absorption

Rutherford Scattering Experiments Atom He “bullet”

Hydrogen Atoms  Simplest atom  One proton, one electron  Most abundant atom  90% of universe is hydrogen

Planetary Model Force p e-e- Since electron orbits the proton, a force exists.

Planetary Model p e-e- Acceleration Force implies acceleration Accelerating charges emit light Light carries energy (E = hf)

Planetary Model p e-e- Electron moves closer to the nucleus since it requires less energy to be there.

Planetary Model  But the electron is still accelerating  Must still be radiating energy (light)  Must move still closer to the nucleus  Electron will spiral into and collide with the nucleus (in about seconds) Atoms do not exist!!

Bohr’s Hypotheses  Stable electron orbits exist where the electron does not lose energy.

Electrons can be here or here but not here p

Bohr’s Hypotheses  Transitions can occur between orbits so long as the electron ends up with the energy of the new level.

Hydrogen EnergyEnergy Å photon Absorption

Hydrogen EnergyEnergy Å photon Emission

+P nm 700 nm 400 nm -e Hydrogen Atom 1 -e -e 5 -e 486 nm 434 nm -e 410 nm

Emission and Absorption Lines  Lines come from electron transitions  Energy change either comes from (absorption) or is given to (emission) photon. E  Photon energy  Frequency  E = hf  Frequency  1/(Wavelength)  f = c/  Wavelength means COLOR

Hydrogen Spectrum EnergyEnergy Lyman (UV) Balmer (VIS) Paschen (IR) Brackett (Far IR)

Aluminum Oxygen Argon Calcium Carbon Helium Hydrogen Iron Krypton Magnesium Neon Nitrogen Sulfur Sodium Xenon

Emission & Absorption Spectra for any Element  nts/Elements.html

Review

End of Light