Photoelectric effect = Release of electrons from metal by light

• Classical theory can explain it by the force on the electrons by the oscillating electric and magnetic field in

light • But it cannot explain

1. for a given metal, the frequency of light wave must be greater than a threshold value

2. electrons are promptly emitted even with very low intensity light

3. the maximum kinetic energy of the electrons does not depend on the intensity of light

Photoelectric Effect

W

K

Kmax

hf

Bound electrons

Free electrons

W=work function of metal

K=kinetic energy of photoelectron

hf=W+Kmax

hfthreshold =W

photoelectric effect

The miracle year 1905

• During this year, Einstein was a clerk in a Swiss pattern office. He published papers on

• 1. the photoelectric effect

• 2. special theory of relativity

• 3. Brownian motion

Garage Door Opener

Charge Couple Device

Photovoltaic cell

Wave Particle Duality

double slit experiment simulation

Matter Wave (de Broglie)

p

h

mp 2

2

1c

mp

50g Tennis ball at 20m/s , λ=6.63×10-34mWave nature cannot be detected

Electron of kinetic energy 50eV, λ=3.9×10-10 mCan be detected sending electron beam through crystal

Examples

The uncertainty principle (Heisenberg) • In classical physics,

determination of a particle’s orbit requires knowing precisely its initial position and momentum (Δx=0;Δpx=0)

• In nature, this knowledge can never be obtained because

ΔxΔpx~h ΔyΔpy~h ΔzΔpz~h

The Uncertainty Principle Illustrated

y

x

y

p

p

x

y

p

p

y

Δyθ θ

hpp

hpyp x

xxy

yp

xp