1. Physics at the end of XIX Century Major Discoveries of XX Century
and
Major Discoveries of XX Century
Thompson’s experiment (discovery of electron)
Emission and absorption of light
Spectra:
Continues spectra
Line spectra
Three problems:
“Ultraviolet catastrophe”
Photoelectric effect
Michelson experiment

2. Thompson’s experiment - discovery of electron (review)
Electron gun
Velocity selector + - + - V

3. Continues spectra and “Ultraviolet catastrophe”
Stefan-Boltzmann law for blackbody radiation:
Wien displacement law:
Rayleigh’s law:
Plank’s law:
Plank’s constant:

4. Example 1: What are the wavelength and the frequency corresponding to the most intense light emitted by a giant star of surface temperature 5000 K?
Example 2: What are the wavelength and the frequency of the most intense
radiation from an object with temperature 100°C?

5. Photoelectric effect Experiment:
If light strikes a metal, electrons are emitted.
the effect does not occur if the frequency of the light is too low
the kinetic energy of the electrons increases with frequency
light A
Classical theory can not explain these results.
If light is a wave, classical theory predicts:
Frequency would not matter
Number of electrons and their energy should increase with intensity
Quantum theory:
Einstein suggested that, given the success of Planck’s theory, light must be emitted and absorbed in small energy packets, “photons” with energy:
If light is particles, theory predicts:
Increasing intensity increases number of electrons but not energy
Above a minimum energy required to break atomic bond, kinetic energy of electrons will increase linearly with frequency
There is a cutoff frequency below which no electrons will be emitted, regardless of intensity

6. Photoelectric effect (quantum theory)
light
Photoelectric effect (quantum theory)
Photons!
Plank’s constant: A
(1) I
(2) V
-V0 V0 f
fmin

7. Example: The work function for a certain sample is 2.3 eV. What is the
stopping potential for electrons ejected from the sample by 7.0*1014 Hz
electromagnetic radiation?
Example: The work function for sodium, cesium, copper, and iron are 2.3, 2.1, 4.7, and 4.5 eV respectively. Which of these metals will not emit electrons when visible light shines on it?
Copper, and iron will not emit electrons

8. Example: Rank the following radiations according to their associated
photon energies, greatest first:
yellow light from a sodium vapor lamp
a gamma ray emitted by a radioactive
a radio wave emitted the antenna of a commercial radio station
a microwave beam emitted by airport traffic control radar
Example: At what rate are photons emitted by a 100 W sodium vapor lamp if we can assume the emission is entirely at a wave-length of 590 nm?