2. Failures of Classical Mechanics Flashcards
Briefly describe the photoelectric effect, and CM’s incorrect assumption
Only light of a sufficiently short wavelength could liberate electrons when illuminated upon a clean metal surface.
It did NOT depend on intensity (CM assumption)
How did Einstein explain the photoelectric effect?
Light was quantised - Individual photons deliver discrete amounts of energy, and 1-2-1 interaction
How many electrons could be liberated if all photons had an energy below the work function?
0
State the equation for the kinetic energy of the photons
E_K = hv - phi
KE of photons = photon energy - work function
What is a photocathode?
A material used for electron emission through illumination
Define quantum yield/efficiency
The number of electrons emitted per incident photon at any given wavelength (Energy)
A yield of 1 (100%)
Describe X-ray production
Bombard electrons into a metal target to liberate photons (reverse photoelectric effect sort of)
What does the classical theory predict about x-ray production?
The wavelengths would be continuous. Instead there is a definite minimum
What does the minimum wavelength correspond to in x-ray production? (And state the equation)
The maximum energy
E_max = hc/λ_min
Max E = hc / minimum wavelength
What has been assumed about the generation of photons in this chapter?
They are generated by INIDIVUDAL events involving an electron rather than an accumulation of energy
What is the energy range for soft x-rays?
hundreds of eV to tens of keV
Describe the intensity-wavelength graph for x-rays
- Sharp increase to peak, then a tail off
- The maximum intensity shifts to shorter wavelengths as the energy (or temp) increases
- First picture on document
Describe compton scattering
The idea that the inelastic scattering of x-rays passing through a thin metal film can be described by a photon-electron collision
How can you find the altered wavelength of the inelastically scattered photons?
By conserving relativistic momentum and energy
State the compton scattering equation
λ’ - λ = h/(m_e * c) * (1 - cos(phi))
Scattered wavelength - incident wavelength = h/(m_e*c) * (1 - (cos(scattering angle))
Describe the wavelengths of cooler and warmer black bodies
Cooler - red (longer wavelengths)
Warmer - blue (shorter wavelengths)
What does Planck’s radiation law describe?
The spectral emmitance of a black body
Describe the graph illustrating Planck’s radation law
- Sharp increase to peak, then a tail off
- The maximum intensity shifts to shorter wavelengths as the temp increases
- Second picture on document
When does electron diffraciton occur, and what phenomena allows for this?
When the wavelength is similar to or shorter than the length scale of the scatter.
Wave-particle duality allows for this
Why can electrons diffract from rays of atoms?
- Atoms act as scatterers
- The wavelength of the electrons is easily tuned to atomic dimensions by choosing energies from tens to thousands of eV (easily attainable in labs)
