Bohr’s Third Postulate

A single photon is emitted whenever an electron jumps down from one orbit to another.

lowerupper EEhf

1. Why do astronomers often use the terms color and temperature interchangeably when referring to stars?

2. Why did Bohr assume that the electrons do not radiate when they are in the allowed orbits?

Wave Nature of Matter

De Broglie Wavelength

mvh

Wave-particle duality apply also to matter.

De Broglie’s Hypothesis Applied to Atoms

Quantum Mechanics of Atoms

The Wave Function and Its Interpolation

In quantum mechanics the amplitude of a particle wave is called the wave function and is given the symbol

If we are dealing only with one photon:

At any point the square of the electric field strength is a measure of the probability that a photon will be at that location.

For a single particle:

2 at a certain point in space and time represents the probability of finding the electron at the given position and time.

Important: there is no way to predict where one electron would hit the screen.

We could predict only probabilities.

The main point of this discussion is this: if we treat electrons as if they were waves, then represents the wave amplitude. If we treat them as particles, then we must treat them on probabilistic basis.

The Heisenberg Uncertainty Principle

The act of observing produces a significant uncertainty in either the position or the momentum of electron.

sJ

hpx

341006.12

Position uncertainty of a baseball

What is the uncertainty in position, imposed by the uncertainty principle, of a 150-g baseball thrown at 42+-1 m/s? Should the umpire be concerned? Can he use Heisenberg as an excuse?

Particle in a Box

Baby-Quiz1. If all objects emit radiation, why don’t we see most of them in the

dark?

2. Suppose you were a nineteenth-century scientist who had just discovered a new phenomenon known as Zeta rays. What experiment could you perform to define if Zeta rays are charged particles or e/m waves? Could this experiment distinguish between neutral particles and an e/m wave?

3. If a metal surface is illuminated by light at a single frequency, why don’t all the photoelectrons have the same kinetic energy when they leave the metal’s surface?

4. What property of the emitted electrons depends on the intensity of incident light?What property of the emitted photoelectrons depends on the frequency of incident light?