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Planck’s Quantum Hypothesis
Energy distributed among oscillating electric charges is quantized (discrete amounts) and not continuous.
E=nhf, n=1,2,3,… f-oscillation frequency
Planck’s constant: h=6.626x10 -34^ J s
hf – quantum of energy (smallest energy possible of molecular oscillators)
Einstein’s Photon Theory of Light
Since molecular oscillators are quantized, the light they emit must have quanta of energy. Thus, transmitted light can be considered as tiny massless particles called photons. All photons in an EM wave of fixed (monochromatic) frequency f have the same energy: E=hf
Where h is a Planck’s constant as given above.
Thus, the momentum of a photon is given by:
λ
h c
hf c
E
p
The Photoelectric Effect
Photons as particles; photon interactions
- Photoelectric Effect: photons are absorbed by atoms and knock out electrons
- Photons can be absorbed by atoms and excite electrons to hire energy states
- Compton scattering: Photons can be scattered by electrons
- Pair production: Photon disappears and creates matter (electron and positron). Also, electron and positron can annihilate each other with their total energy (kinetic plus rest energies) appearing as photon energy
Compton Scattering
Pair Production
Wave Particle Duality
In some experiments, light acts like a wave (interference, diffraction) where, in other experiments (see above), it acts like a stream of particles.
E=hf: particle energy E, associate with corresponding wave of frequency f.
Principle of Complementarity
The wave and particle aspects of light “compliment” each other, and we must be aware of both to fully understand light.
