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