Post on 17-Jan-2018
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Quantum MechanicsExperiments
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Photoelectric effect
Photoelectric effect
.h k e
.h k e
Lenard 1902: Studied energy of the photoelectrons with intensity of light.He could increase the intensity thousand fold.
1. Noticed a well defined minimum voltage Vstop to stop the current in the circuit. Vstop was independent of the intensity of light.
Current vs Voltage
Cut-off voltage
Different intensities: Ib > Ia
2. Increasing the intensity of light would increase the current
3. He performed the experiment withvarious coloured lights and found themaximum energy of the electrons diddepend on the frequency of light.Qualitatively he obtained more thefrequency more the energy.
Objections with wave theory
1. Kinetic energy (K) of the photo-electron should increasewith intensity of the beam.
But Kmax was found to beindependent of the intensityof the falling light.
2. Effect should occur for any frequency of light providedonly that light is intense enough to eject the electron.
But a cut-off frequency was observed below which photoelectrons were not ejected (no matter howintense was beam).
3. Energy in the classical theoryis uniformly distributed over the wave front. If light is feeble,there should be a time lag betweenthe light striking the plate andejection of photoelectrons. Ejection is instant, t < 10 sec- 9
Einstein equation
Light is wave : Interference, Diffraction, polarisationLight is a stream of photons/wave packets (particles) So wave behaves like particle
Compton effect
Collision between photon and electron
Arthur Holly Compton(1892-1962)
Compton Effectin 1920
Partial transfer of photon energy
m = m(v) is the relativistic mass
Conservation of momentum along initial photon direction
Conservation of momentum along perpendicular to initial photon direction
Square and add the above two expressions
Energy conservation
Compton wavelength
Compton shift
= 2.4 pm
~
Compton Experiment
Franck and Hertz experiment (1914)(James Franck and Gustav Hertz)
Confirmation of discrete energy levels in atom
Vo
100
200
300
I in mA
Accelerating Voltage in Volts5 10 15
Peaks at 4.9 V and its multiples
Groundstate
1st excitedstate
2nd excitedstate
4.9 eV6.7 eV
Continuum
E=0
E= -10.4eV
Particles behaving as waves
Electron diffractionDavisson –Germer (USA)and Thompson (UK) (1927)
Experiments
Diffraction?
Apply Bragg’s lawFrom X-ray diffraction
KE=54 eV is non relativistic
mEp
ph
2
de Broglie wavelength of electron
mEp
ph
2
“In a situation where the wave aspect of a system is revealed, its particle aspect is concealed; and,in a situation where the particle aspect is revealed, its wave aspect is concealed. Revealing both simultaneously is impossible; the wave and particle aspects are complementary.”
Bohr’s Complementarity Principle