Lecture 34: The Wonderful Quantum World - Breakdown of Classical Mechanics
recorded by: Massachusetts Institute of Technology, MIT
published: Oct. 10, 2008, recorded: December 1999, views: 12383
released under terms of: Creative Commons Attribution Non-Commercial Share Alike (CC-BY-NC-SA)
Download mit801f99_lewin_lec34_01.m4v (Video - generic video source 102.0 MB)
Download mit801f99_lewin_lec34_01.rm (Video - generic video source 104.2 MB)
Download mit801f99_lewin_lec34_01.flv (Video 102.2 MB)
Download mit801f99_lewin_lec34_01.wmv (Video 419.0 MB)
Report a problem or upload filesIf you have found a problem with this lecture or would like to send us extra material, articles, exercises, etc., please use our ticket system to describe your request and upload the data.
Enter your e-mail into the 'Cc' field, and we will keep you updated with your request's status.
1. Discrete Energy Levels:
Electrons orbit their atomic nucleus in well defined orbits corresponding to discrete energy levels. The electrons can jump from one energy level to a vacant energy level, but they cannot exist in between. Transitions between these energy levels gives rise to absorption and emission of light in discrete spectral lines (wavelengths). The students are encouraged to look through their diffraction gratings at helium and neon light sources to see evidence of these discrete wavelengths of emitted light.
2. Particles and Waves:
Quantum mechanics introduces some very non-intuitive concepts, e.g. light behaves as both a particle (a photon) and a wave, and a particle behaves like a wave with a wavelength inversely proportional to its momentum. Interference is a wave phenomenon, and indeed particles can interfere with each other. Both the position and momentum of a particle cannot be accurately specified at the same time (Heisenberg's uncertainty principle).
3. Diffraction by a Slit:
Diffraction of light by a narrow vertical slit is a well understood classical wave phenomenon consistent with Heisenberg's uncertainty principle. The narrower the slit, the smaller is the uncertainty in the horizontal position of the photons which have to sneak through the narrow opening, so the greater is the horizontal spread of the transmitted protons (uncertainty in their momentum). Quantum mechanics only allows you to predict positions of particles with certain probabilities. In the classical, Newtonian, world you can predict the position and movement of a particle to any degree of accuracy - NOT in the microscopic quantum world. The Newtonian picture is perfect for describing the behaviour of basketballs and planets in the macroscopic world.
Link this pageWould you like to put a link to this lecture on your homepage?
Go ahead! Copy the HTML snippet !