27.1 The Electron Cathode rays are light waves or particles?

1. 27.1 The Electron • Cathode rays are light waves or particles? APHY201

2. 27.1 The Electron • 1897 – J. J. Thomson measures e/m directly using electric and magnetic fields. • Newton’s 2nd Law: F = ma • Centripetal Motion: a = v2/r • Magnetic Force: FB = evB • Electric Force: FE = eE • Millikan oil-drop experiment APHY201

3. 27.2 Planck’s Quantum Hypothesis • Light from hot dense objects - Blackbody radiation λpT = 2.9 x 10-3 m K APHY201

4. 27.2 Planck’s Quantum Hypothesis APHY201

5. 27.2 Planck’s Quantum Hypothesis APHY201

6. 27.2 Planck’s Quantum Hypothesis • Max Planck (1900) theorized that atoms in molecules have oscillations with discrete or quantized energies given by En = nhf where h = 6.6 x 10-34 Js • A quantized world is grainy and coarse which is not noticeable in our everyday experience • Example: a mass oscillating on a spring APHY201

7. 27.2 Planck’s Quantum Hypothesis APHY201

8. 27.3 Photon Theory of Light • Einstein suggested that light is quantized or made of photons • A photon is emitted with an energy E = hf when a molecule’s energy state changes from a higher level to a lower level. • Photoelectric Effect APHY201

9. 27.3 Photon Theory of Light • Classical wave theory of light cannot explain why … • Higher intensity light should eject electrons of higher KE. A single photon is absorbed to eject the electron • There is no relationship between KE and frequency. KE increases linearly with increasing frequency • The photoelectric effect should be observed at any frequency. No electrons are ejected below fo for the material • The electrons require time to absorb enough energy to be ejected from the metal. The electrons are ejected in less than 10-9 s APHY201

10. 27.5 The Compton Effect • 1923: X-rays scattered off electrons in graphite APHY201

11. 27.6 Photon Interactions • Photoelectric effect, excitation of atoms, Compton scattering, pair production/annihilation Minimum energy is 1.02 MeV Gamma ray photons APHY201