# APHY201 4/29/2015 1 27.1 The Electron   Cathode rays are light waves or particles?

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APHY201 4/29/2015 1 27.1 The Electron   Cathode rays are light waves or particles?

APHY201 4/29/2015 2 27.1 The Electron   1897 – J. J. Thomson measures e/m directly using electric and magnetic fields.   Newton’s 2 nd Law: F = ma   Centripetal Motion: a = v 2 /r   Magnetic Force: F B = evB   Electric Force: F E = eE   Millikan oil-drop experiment

APHY201 4/29/2015 3 27.2 Planck’s Quantum Hypothesis   Light from hot dense objects - Blackbody radiation λ p T = 2.9 x 10 -3 m K

APHY201 4/29/2015 4 27.2 Planck’s Quantum Hypothesis

APHY201 4/29/2015 5 27.2 Planck’s Quantum Hypothesis

APHY201 4/29/2015 6 27.2 Planck’s Quantum Hypothesis   Max Planck (1900) theorized that atoms in molecules have oscillations with discrete or quantized energies given by E n = 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 4/29/2015 7 27.2 Planck’s Quantum Hypothesis

APHY201 4/29/2015 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 4/29/2015 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 f o 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 4/29/2015 10 27.5 The Compton Effect   1923: X-rays scattered off electrons in graphite

APHY201 4/29/2015 11 27.6 Photon Interactions   Photoelectric effect, excitation of atoms, Compton scattering, pair production/annihilation Minimum energy is 1.02 MeV Gamma ray photons

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