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Wave-Particle Duality
• e/m radiation exhibits diffraction and interference
=> wave-like
• particles behave quite differently - follow well
defined paths and do not produce interference
• when  << size of opening, wave behaves like a
• light exchanges energy in “lumps” or ‘quanta’ just
like particles
Water waves flare out when passing through opening of width a
Wave-Particle Duality
1900: sound, light, e/m radiation were waves
electrons, protons, atoms were particles
1930: quantum mechanics provided a new interpretation
light behaves as a particle: photoelectric & Compton effect
E=hf = hc/
particles behave as waves: electron diffraction
=> localized packets of energy => particle-like
f,  “wave-particle duality”
Double Slit Experiment with electrons (1989)
Modern Physics
Large objects
Large objects
small speeds
large speeds
“Newtonian Physics”
“relativistic mechanics”
F = ma
F = dp/dt
Atomic scales
small speeds
Quantum Mechanics
Atomic particles
Large speeds
relativistic quantum
“Dirac Equation”
Electromagnetic Waves
• Maxwell(1860) showed that light is a
travelling wave of electric and magnetic
• E = Em sin (kx-t)
• B = Bm sin (kx-t)
• v= /k = c ~ 3 x 10 8 m/s
• the speed is the same in all reference frames
• v= c/n in material media ( n=1 for vacuum)
Transverse Wave
E and B are both  to v and E  B
• Light is a wave
• => exhibits interference and diffraction
• => oscillating electric and magnetic fields are
solutions of Maxwell’s equations
• => Maxwell’s equations predict a continuous range
of ’s from -rays to long radio waves
• electromagnetic spectrum
Electromagnetic Spectrum
Power  2
Sensitivity of eye to various 
• heated objects “glow” if the temperature is
high enough
• =>embers in a fire, stove element
• => bar of steel heated to 12000 K glows in
deep red colour
• thermal radiation
• charges in material vibrate in SHM(accelerate) and
produce e/m radiation
• also occurs at lower T but  is longer => infra-red
and not visible
Classical prediction
for 14500 K
Cannot explain the peak
R ( , T ) 
2 ckT
Watts m-2s-1
As T decreases,  of peak increases
Partially explained by Planck 1900
R ( , T ) 
2 c 2 h
ehc /  kT  1
Modern Physics
• 1905 Einstein proposed:
• when an atom emits or absorbs light, energy
• is not transferred in a smooth continuous
fashion but rather in discrete “packets” or
“lumps” of energy
• “photons” have energy E=hf
Planck’s constant
h=6.63x10-34 J.s
Modern Physics
• h plays a similar role to c in relativity
• if c   then no relativity! v/c <<1 always
=> signals transmitted instantaneously
• if h  0
then no quantum mechanics
=> no stable atoms!
• Consider a 100W sodium vapour lamp with
= 590 nm
• what is the energy of a single photon?
• E=hf = hc/
=(6.63x10-34 J.s)(3x108 m/s)/590x10-9 m)
= 3.37x10-19 J
• Power = dE/dt
=[number of photons/sec] x 3.37x10-19 J
= 100 W
• number of photons/sec = 3 x 1020
• The amount of sunlight hitting the earth is about
1000 W/m2 and  ~ 500 nm
• photons/sec/m2 ~ 2.5x 1021
• we do not see the grainy character of the energy
distribution => appears continuous
• photoelectric effect (lab 4)
• if we shine a beam of light of short enough  onto a
clean metal surface, the light will knock electrons
out of the metal surface