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