the photon a “particle” of light a “quantum” of light energy the energy of a given photon...
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The photon•A “particle” of light•A “quantum” of light energy•The energy of a given photon depends on the frequency (color) of the light
But light is also a wave!
•Travels at constant speed c in a vacuum.
•c = f–c: 3 x 108m/s– wavelength (m)– f: frequency (Hz)
Calculating photon energy
•E = hf–E: energy (J or eV)–h: Planck’s constant
•6.62510-34 J s or 4.14 10-15 eV s
–f: frequency of light (s-1, Hz)
The “electron-volt” (eV)is an energy unit
•Useful on the atomic level.•If a moving electron is stopped by 1 V of electric potential, we say it has 1 electron-volt (or 1 eV) of kinetic energy!
Converting eV to Joules (J)
1 eV = 1.60210-19J
Photoelectric Effect experiment
Photo-Diode
(+)
A
V
Collector (-)
e- e- e- e- e- e-
e-
e-
e-
e-e-e-e-e-e-
e-
e-
At a certain voltage, Vs, the current can’t flow anymore!
light
e- e- e- e-e- e- e- e- e-
light
Anomalous Behavior in Photoelectric Effect
• Voltage necessary to stop electrons is independent of intensity (brightness) of light.
• Photoelectrons are not released below a certain frequency, regardless of intensity of light.
• The release of photoelectrons is instantaneous, even in very feeble light, provided the frequency is above the cutoff.
Voltage current for different intensities of light.
V
i
Vs
I1
I2
I3I3 > I2 > I1
Stopping potential is unaffected!
Voltage versus current for different frequencies of light.
V
i
Vs,1
f1f2
Vs,2
f3
Vs,3
f3 > f2 > f1
Stopping potential becomes more negative at higher frequencies!
Photoelectric Effect
• Ephoton = Kmax + Wo
–Ephoton = hf (Planck’s equation)
–Kmax: maximum kinetic energy of electrons
–Wo: binding energy or “work function”
• hf = Kmax + Wo
Graph of Photoelectric Equation
f
Kmax
hf = Kmax+ Wo
Kmax = hf - Wo
y = mx + b
slope = h(Planck’s Constant)
Wo(binding energy)
Cut-off frequency
Absorption SpectrumPhoton is absorbed and excites atom to higher quantum energy state.
0 eV
-10 eV
hf
Ground state
E
Absorption SpectrumAbsorption spectra always involve atoms going up in energy level.
0 eV
-10 eV
ionized
Emission SpectrumPhoton is emitted and atom drops to lower quantum energy state.
0 eV
-10 eV
hf
Excited state
E
Emission SpectrumEmission spectra always involve atoms going down in energy level.
0 eV
-10 eV
ionized
A typical nucleus
C12
6
Element name
Atomic mass: protons plus neutrons
Atomic number: protons
Isotope characteristics differ
U238
92U
235
92
Binding energy
• Energy released when a nucleus is formed from protons and neutrons.
• Mass is lost.
• E = mc2
–where m is the lost mass
Nuclear Particles
• Nucleons– Proton
• Charge: +e• Mass: 1 amu
– Neutron• Charge: 0• Mass: 1 amu
p11
n10
Nuclear reactions• Nuclear Decay
–Alpha decay–Beta decay
• Beta Minus• Positron
• Fission• Fusion
Decay Particles
•Alpha
•Beta •Positron
He4
2
e0-1
e01
Alpha Decay• Occurs only with very heavy
elements.• Nucleus too large to be stable.
Rn222
86Ra
226
88He4
2
Beta Decay• Occurs with elements that have
too many neutrons for the nucleus to be stable.
Ca40
20K
40
19e0
-1
anti-neutrino
Positron Decay• Occurs with elements that have
too many protons for the nucleus to be stable.
H 2
1He
2
2e0
1
neutrino
Neutrino and Anti-Neutrino
• Proposed to make beta and positron decay obey conservation of energy.
• No mass, no charge.
• Energy and spin.
• Does not react easily with matter.
• Hard to detect.
Gamma Radiation,
• Released by atoms which have undergone a nuclear reaction.
• Results when excited nuclei return to ground state.
• High energy! E = hf!
Fission• Occurs only with very heavy
elements.
• Nucleus too large to be stable.
• Induced by neutrons.
Sr92
38Pu
239
94n1
0n1
04Ba
144
56
Fusion• The largest amount of energy available.
• Energy produced in the sun.
• Fusion of light elements results in non-radioactive waste.
He 2
2H
1
1H
1
1
Summary of Wave-Particle Duality
Waves are particles and particles are waves
Energy
• Particle–E = K + U
• Photon–E = hf
Momentum
• Particle–p = mv
• Photon–p = h/
Wavelength
• Photon– = c/f
• Particle– = h/p
– deBroglie wavelength
Compton Scattering
• Proof of the momentum of photons.• High-energy photons collided with
electrons.• Conservation of momentum.• Scattered photons examined to
determine loss of momentum.
Davisson-Germer Experiement
•Verified that electrons have wave properties by proving that they diffract.
•Electron diffraction