Modern Physics Lecture III. The Quantum Hypothesis. In this lecture we examine the evidence for “light quanta” and the implications of their existence Waves as Particles The photoelectric effect Compton scattering Particles as Waves Electron diffraction The Double Slit Revisited.
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Modern PhysicsLecture III
Photoelectric effect
Tungsten - wolfram
Recoiling electron
f
q
Incident
Photon, l0
Scattered
Photon, l’
But…
K = m v 2/2 = q V
momentum p = m v
We can express kinetic energy in terms of momentum
K = p 2/(2 m) = q V
Reorganise to get
p = (2 m q V )1/2
de Broglie’s hypothesis gives
l= h / p
Substitute for pto get
dNi=0.215nm
diffraction
de Broglie
a, b, c – computer simulation
d - experiment
http://www.chem.qmw.ac.uk/surfaces/scc/scat6_2.htm
Wave Properties
Reflection, Refraction
A property of both particles and waves
Interference and Diffraction
Young’s double slits
Waves Only
Polarisation
Waves Only
Particle properties
Consists of discreet particles
(atoms or molecules)
Momentum
A well defined trajectory
Does not diffract or interfere
1 particle + 1 particle = 2 particles
Conclusion: Our initial hypothesis is incorrect. We need to form a new hypothesis for the differences between matter and light. One could be that matter has a rest mass!