2. Failures of Classical Mechanics

Question 1

Briefly describe the photoelectric effect, and CM’s incorrect assumption

Answer 1

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)

Question 2

How did Einstein explain the photoelectric effect?

Answer 2

Light was quantised - Individual photons deliver discrete amounts of energy, and 1-2-1 interaction

Question 3

How many electrons could be liberated if all photons had an energy below the work function?

Answer 3

0

Question 4

State the equation for the kinetic energy of the photons

Answer 4

E_K = hv - phi

KE of photons = photon energy - work function

Question 5

What is a photocathode?

Answer 5

A material used for electron emission through illumination

Question 6

Define quantum yield/efficiency

Answer 6

The number of electrons emitted per incident photon at any given wavelength (Energy)
A yield of 1 (100%)

Question 7

Describe X-ray production

Answer 7

Bombard electrons into a metal target to liberate photons (reverse photoelectric effect sort of)

Question 8

What does the classical theory predict about x-ray production?

Answer 8

The wavelengths would be continuous. Instead there is a definite minimum

Question 9

What does the minimum wavelength correspond to in x-ray production? (And state the equation)

Answer 9

The maximum energy
E_max = hc/λ_min
Max E = hc / minimum wavelength

Question 10

What has been assumed about the generation of photons in this chapter?

Answer 10

They are generated by INIDIVUDAL events involving an electron rather than an accumulation of energy

Question 11

What is the energy range for soft x-rays?

Answer 11

hundreds of eV to tens of keV

Question 12

Describe the intensity-wavelength graph for x-rays

Answer 12

• 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

Question 13

Describe compton scattering

Answer 13

The idea that the inelastic scattering of x-rays passing through a thin metal film can be described by a photon-electron collision

Question 14

How can you find the altered wavelength of the inelastically scattered photons?

Answer 14

By conserving relativistic momentum and energy

Question 15

State the compton scattering equation

Answer 15

λ’ - λ = h/(m_e * c) * (1 - cos(phi))

Scattered wavelength - incident wavelength = h/(m_e*c) * (1 - (cos(scattering angle))

Question 16

Describe the wavelengths of cooler and warmer black bodies

Answer 16

Cooler - red (longer wavelengths)

Warmer - blue (shorter wavelengths)

Question 17

What does Planck’s radiation law describe?

Answer 17

The spectral emmitance of a black body

Question 18

Describe the graph illustrating Planck’s radation law

Answer 18

• Sharp increase to peak, then a tail off
• The maximum intensity shifts to shorter wavelengths as the temp increases
• Second picture on document

Question 19

When does electron diffraciton occur, and what phenomena allows for this?

Answer 19

When the wavelength is similar to or shorter than the length scale of the scatter.
Wave-particle duality allows for this

Question 20

Why can electrons diffract from rays of atoms?

Answer 20

• 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)