properties of light the speed of light galileo’s experiment a b c = round trip distance/total time
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Properties of LightProperties of Light
The Speed of LightThe Speed of Light
Galileo’s Experiment
AB
c = Round Trip Distance/Total Time
Ole Roemer (1675)Ole Roemer (1675)
Earth
Jupiter
Speed of LightSpeed of Light
From Roemer’s Experimentc = Diameter of Orbit/Time Delay
Modern Valuec = 299,792.458 km/sec
Speed of LightSpeed of Light
Absolute constant
Nothing travels faster
Relative VelocitiesRelative VelocitiesOur 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
Vrel = 0v
v
v v Vrel = 2v
Try it with LightTry it with Light
Sun
½ c
c
Intuition says you get ½ cEinstein says c
Try it with Light
Sun½ c
c
Intuition says you get 1½ cEinstein says c
Nature of LightNature of Light
Particle?Newton
Wave?Christian Huygens in Newton’s dayThomas Young (1801)
Light can diffract - must be a waveTransverse wave
Types of WavesTypes of Waves
Transverse and Longitudinal
WavelengthWavelength
Electromagnetic Electromagnetic SpectrumSpectrum
Increasing Wavelength
Visible Light
The Visible SpectrumThe Visible Spectrum
400nm 500nm 600nm 700nm
Wavelength means
COLOR
FrequencyFrequency
Number of complete events occurring in a period of timeWaves/second
ExamplesSecond hand on a clock?
1 cycle/minute = 1/60 cycle/secUS Presidential Electons
1 election/4 years = 1/4 election/year
Frequency and PeriodFrequency and Period
Period =
1
Frequency
Speed, Wavelength, Speed, Wavelength, FrequencyFrequency
c = f
Photon EnergyPhoton Energy
E f
E = hf
Properties of LightProperties of LightSpeed (c)
Nothing travels faster
Absolute constantWavelength ()
Gamma rays, X-rays, UV, Visible, IR, Radio
In visible wavelength means colorFrequency (f)
Number of complete waves per secondEnergy of a Photon (E)
E = hf
Dispersive Element
Basic SpectrographBasic Spectrograph
Collimating Lens
Recording Device
Slit
Imaging Lens
Mercury near HorizonMercury near Horizon
The atmosphere can act like a prism
Continuous SpectrumContinuous Spectrum
Source must be HOT and DENSE
Emisson (Bright Line) Emisson (Bright Line) SpectrumSpectrum
Source must be HOT and TENUOUS
Absorption SpectrumAbsorption Spectrum
Types of SpectraTypes of Spectra
Continuous
Emission
Absorption
Rutherford Scattering Rutherford Scattering ExperimentsExperiments
Atom
He “bullet”
Hydrogen AtomsHydrogen Atoms
Simplest atomOne proton, one electron
Most abundant atom90% of universe is hydrogen
Planetary ModelPlanetary Model
Force
p
e-
Since electron orbits the proton, a force exists.
Planetary ModelPlanetary Model
p
e-
Acceleration
•Force implies acceleration
•Accelerating charges emit light
•Light carries energy (E = hf)
Planetary ModelPlanetary Model
p
e-
Electron moves closer to the nucleus since it requires less energy to be there.
Planetary ModelPlanetary Model
But the electron is still acceleratingMust still be radiating energy (light)Must move still closer to the nucleusElectron will spiral into and collide with
the nucleus (in about 10-8 seconds)
Atoms do not exist!!
Bohr’s HypothesesBohr’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 HypothesesBohr’s Hypotheses
Transitions can occur between orbits so long as the electron ends up with the energy of the new level.
HydrogenHydrogen
Energy
2
345
6563 Å photon
Absorption
HydrogenHydrogen
Energy
2
345
6563 Å photon
Emission
+P+P
22
33
44
656 nm
700 nm700 nm400 nm400 nm
-e-e
Hydrogen Hydrogen AtomAtom
11
-e-e
-e-e
55
-e-e
486 nm
434 nm
-e-e410 nm
Emission andEmission andAbsorption LinesAbsorption Lines
Lines come from electron transitionsEnergy change either comes from
(absorption) or is given to (emission) photon. E
Photon energy FrequencyE = hf
Frequency 1/(Wavelength)f = c/
Wavelength means COLOR
Hydrogen SpectrumHydrogen Spectrum
Energy
1
2
34
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 Emission & Absorption Spectra for any ElementSpectra for any Element
http://jersey.uoregon.edu/vlab/elements/Elements.html
Review
End of LightEnd of Light