Thompson’s experiment (discovery of electron )

1. Physics at the end of XIX 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. Continues spectra and “Ultraviolet catastrophe” Stefan-Boltzmann law for blackbody radiation: Wien’s displacement law: I(λ) Plank’s constant:

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

4. Photoelectric effect light A 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 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 kinetic energy • Above a minimum energy required to break atomic bond, kinetic energy will increase linearly with frequency • There is a cutoff frequency below which no electrons will be emitted, regardless of intensity

5. light A Photoelectric effect (quantum theory) Photons! Plank’s constant: Stopping potential (V0): I V -V0

6. 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? Visible light: Copper, and iron will not emit electrons

7. The Atom 1. The Thomson model (“plum-pudding” model) It was known that atoms were electrically neutral, but that they could become charged, implying that there were positive and negative charges and that some of them could be removed. This model had the atom consisting of a bulk positive charge, with negative electrons buried throughout. Later, Rutherford did an experiment that showed that the positively charged nucleus must be extremely small compared to the rest of the atom.

8. 2. Rutherford’s scanning experiment and planetary model Rutherford scattered alpha particles – helium nuclei – from a metal foil and observed the scattering angle. He found that some of the angles were far larger than the plum-pudding model would allow. Rutherford’s (planetary) model: The only way to account for the large angles was to assume that all the positive charge was contained within a tiny volume – now we know that the radius of the nucleus is about 1/100000 that of the atom.

9. 3. Atomic line spectra (Key to the structure of the atom) A very thin gas heated in a discharge tube emits light only at characteristic frequencies. • An atomic spectrum is a line spectrum – only certain frequencies appear. • If white light passes through such a gas, it absorbs at those same frequencies.

10. 4. Hydrogen atom The wavelengths of electrons emitted from hydrogen have a regular pattern: Rydberg constant: A portion of the complete spectrum of hydrogen: These lines cannot be explained by the Rutherford theory

11. 5. The Bohr Atom Bohr proposed that the possible energy states (stationary states) for atomic electrons were quantized – only certain values were possible. Then the spectrum could be explained as transitions from one level to another. Example:

12. The Bohr Atom The lowest energy level is called the ground state; the others are excited states.

13. Example: Franck- Hertz experiment Franck and Hertz studied the motion of electrons through mercury vapor under the action of an electric field. When the electron kinetic energy was 4.9eV or grater, the vapor emitted ultraviolet light. What was the wave length of this light?