1. Instructor: Suchetana Chatterjee Presidency University Physics -0402
Quantum Mechanics I
Lecture 1
Instructor: Suchetana Chatterjee
Presidency University
Physics -0402

2. Triumphs of Classical Physics
Nineteenth century Physics:
Newton’s laws of motions,
Newton’s law for gravitation,
Wave optics,
Laws of thermodynamics,
Electromagnetism
(unified electricity and magnetism with optics)
Motion of planets
Astronomy
Industrial Revolution

3. Late nineteenth century
“Necessity is the mother of invention”
Late nineteenth century
Photoelectric effect
The photoelectric effect is the observation that many metals emit electrons when light shines upon them. Electrons emitted in this manner may be called photoelectrons.
….. Wikipedia

4. Try to explain it with 19th century Physics
Light is an electromagnetic wave..
Natural explanation is light provides energy to the electrons
Light model prediction I
The intensity of EM wave depends on its amplitude and not on its frequency. So the magnitude of the photocurrent should depend on the intensity of the wave.
Light model prediction II
It takes a certain minimum energy to eject one electron, known as the work function. According to E&M theory energy is distributed evenly throughout the wave.
Hence for a very dim light, it would take some time for enough light energy to build up to eject an electron from a metallic surface. Hence one would expect a lag between when we switch on the light and when photo electrons appear.
Light model prediction III
The stopping potential should depend on intensity and not on frequency.

5. What’s wrong???? Experimental Observations
The photocurrent depends on the light frequency. For a given material monochromatic light with a frequency below a minimum threshold frequency produces no photo-current regardless of the intensity.
There is no measurable time delay between when the light is turned on and when the cathode emits photoelectrons. No matter how faint the light is.
The stopping potential does not depend on intensity, but does depend on frequency.
What’s wrong????

6. Thermal Radiation (late 19th century)
The radiation emitted by a body as a virtue of its temperature is called thermal radiation
Matter in a condensed state emits a continuous spectrum of radiation.
What is thermal radiation?
Random motion of molecules, collisional process.
Depends on the temperature
“A black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence.” ……………..Wikipedia
Lummer & Pringsheim 1899
using prism spectrometers.

7. Classical theory of cavity radiation
Spectral Radiancy: Energy emitted per unit time in radiation of frequency in the interval
n to n+dn from a unit area of the surface of a blackbody at absolute temperature T.
Classical theory of cavity radiation
Rayleigh and Jeans
Classical theory of electromagnetic radiation was used to construct the theory.
Radiation in the cavity must exist in the form of standing waves with nodes in the metallic surfaces.
Count the number of nodes.
Use kinetic theory to get the energy of these waves assuming thermal equilibrium. (Will do as a HW problem)

8. Ultraviolet catastrophe: Experiment and Theory disagrees.
What’s wrong????

9. Atomic structure What’s Wrong????
J. J. Thompson in 1897 gave the model of the atomic nucleus.
Rutherford scattering experiments (~ 1910) confirmed the existence of the atomic nucleus.
Existence of line emission and absorption (discrete frequencies)
Failure of classical Physics: Rutherford’s discovery of the atomic nucleus raised a serious question.
What prevented the negatively charged electrons from falling into the positively charged nucleus due to strong electrostatic attraction?
Any accelerating charge radiates EM waves. The energy of an orbiting electron should decrease continuously. Its orbit should become smaller and smaller. The atom will not be stable !!!!!
The frequency of the EM wave should be equal to the frequency of revolution. They should emit continuous spectrum.
What’s Wrong????

10. Gave Birth to the notion of Quantum Reality
Our notion of classical physics based on classical electromagnetism, thermodynamics,
Newtonian mechanics fail to reproduce these observations.
Something different than our view of the classical world is happening here. All these problems were solved with radical views of physical reality.
a) Energy quantization
b) Wave-particle Duality
Gave Birth to the notion of Quantum Reality