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

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

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

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?

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

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

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

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?