EMR 2 The Compton Effect

Review of Photoelectric Effect: Intensity Matters - the greater the intensity/brightness, the greater the photoelectric current Because.... One photon emits one electron Dim light – 3 photons hit metal and emit 3 electrons Bright light – 5 photons hit metal and emit 5 electrons so... there is a greater photoelectric current

Robert Millikan Irradiated different substances with different frequencies of radiation to determine f o MetalF o x Hz Lead2.5 Zinc2.0 Copper4.0

Graphs 1. Energy vs frequency 2. Current vs frequency fofo W Slope = h

Graphs 3. Current vs Intensity 4. Current vs Stopping Voltage

Wave Particle Duality Theory Theory indicates that particles can behave as waves and waves can behave as particles. To describe the photoelectric effect, light or radiation is described as a wave and can be considered a particle because it has energy Energy is a particle-like property

Compton Effect Einstein predicted that photons should also possess momentum (particle like property) Two equations that describe the momentum of photons making no reference to mass

Examples Determine the momentum of a a. Photon of wavelength 250 nm b. Electron moving at 4.00 x 10 5 m/s

Compton (1922) – indicated experimentally that photons have momentum directed x-rays at graphite (carbon) atoms and detected the scattered rays Incident beam Graphite or carbon atom Scattered x-ray

the energy and momentum gained by the electron within the atom equals the energy and momentum lost by the photon the results obey the laws of conservation of energy and momentum Compton Effect – the scattering of an x-ray by an electron resulting in a reduced frequency of the x-ray (increase in wavelength)

Momentum of an x-ray (EMR) e Scattered x-ray Compton showed how the change in wavelength ( ) of the scattered photon is related to the angle at which the x-ray is scattered

Equation Energy and momentum are conserved, so the collision is elastic Speed of light Mass of scattering electrons Angle of scattered photon Change of wavelength of photon

Eg) Determine the maximum change in wavelength of a nm x-ray scattered by an electron (maximum scattering is at 180 ⁰ where the photon bounces back) ( occurs when 1 – cos θ is a maximum value at θ = 180 ⁰ )

Eg) An x-ray photon with a wavelength of nm scatters at 60.0 ⁰ after contacting an electron. Determine the wavelength of the scattered photon.