5.3 - Diffraction

Question 1

What is diffraction and what must be met for it to occur

Answer 1

• the spreading out of wave as it passes through a gap
• the size of the gap must be the same magnitude as the wavelength of the wave

Question 2

what happens if the gap is smaller than the wavelength of the wave

Answer 2

the wave will reflect

Question 3

What does electron diffraction provide evidence for

Answer 3

the wave nature of electrons - suggests the particle can demonstrate wavelike properties

Question 4

Describe the diffraction patterns produced by electrons

Answer 4

concentric circles of bright and dark fringes from a central bright point

Question 5

If electrons didn’t have wave nature, describe the pattern that would be seen through

Answer 5

electrons would be unaffected by the gap and pass straight through - a single bright region would be formed

Question 6

What is the basic process of pulse echo technique

Answer 6

• a wave pulse is emitted
• it is transmitted and reflected at the boundary between two media
• the returning echo is detected
• s=d/t used to calculate

Question 7

What two factors can limit the amount of information obtained by pulse-echo

Answer 7

• wavelength of radiation
• duration of the pulse

Question 8

Describe the photoelectric effect

Answer 8

• light is shone onto a metal plate
• if the light has high enough frequency electrons are emitted from surface of metal
• if frequency is too low no electrons are emitted

Question 9

How can a photon liberate and electron

Answer 9

• one photon interacts with one electron and transfers all its energy to it
• if its energy is greater than the metals work function, the electron will have sufficient energy to be released

Question 10

If the intensity of light being shone on an a metal increases, how does the energy of photoelectrons change

Answer 10

• energy remains unaffected
• an increase in intensity means more photons per area and so more photoelectrons are emitted

Question 11

Why are photoelectrons emitted with a range of kinetic energies

Answer 11

• electrons are at different depths in the metal so require different amounts of energy to be liberated
• the excess energy from a photon once and electron has been emitted, is the kinetic energy of the electron

Question 12

What happens when electrons transition between energy levels

Answer 12

• if electrons move to a higher energy level, radiation must be absorbed
• if electrons move to a lower energy level, radiation must be emitted

Question 13

Why can only certain frequencies of radiation be absorbed by an atom to cause electron transition

Answer 13

• the electrons can only exist in discrete energy levels
• the energy of the photon absorbed must be the exact amount of energy required to cover the difference between two discrete energy levesl