Particles of Light

Question 1

What was Planck’s theory for the photoelectric effect?

Answer 1

1. For the electron to be removed from the surface of the metal it must absorb a certain minimum amount of energy
2. Planck suggested that when light falls onto the surface of the metal, one photon interacts with one electrons and that the photons energy is absorbed by the electron
3. The UV photons have enough energy to knock an electron out of the metal so as soon as UV photons hit the since sheet electron are removed from the metal and the electroscope immediately begins to discharge
4. However the photons of visible light have a lower frequency and when they are absorbed by an electron; the electron does not have sufficient energy to escape form the metal surface, so the electroscope doe snot discharge

Question 2

What is the threshold frequency?

Answer 2

The minimum frequency of light required to cause the photoelectric effect

Question 3

Does the threshold frequency change?

Answer 3

• The threshold frequencies varies for different materials
• Visible light remove photoelectrons from alkali metals and calcium and barium
• Other metals require UV radiation which has a higher frequency than visible light
• Electrons at the surface require less energy to escape compared to electrons deeper within the metal, which may be bound more strongly to atoms

Question 4

Can electrons be released by absorbing two photons?

Answer 4

• Electrons cannot be realised by absorbing two photons just as the person cannot escape the hole with two smaller jumps
• Each photon must carry at least the threshold energy in order to release an electron

Question 5

How intensity affect photoelectric effect?

Answer 5

1. More photoelectrons ar emitted as the intensity of light incases, but only if the frequency of light used is above the threshold value
   - intensity is the rate of energy transfer per unit area
2. As light intensity increase, the number of photons increases but the energy per photon stays the same, because there are more photons more electrons can be related so long as the frequency of radiation is above the threshold frequency

Question 6

What are the energies of photoelectrons emitted?

Answer 6

1. Photoelectrons emitted from a particular metal, have a range of energies
2. Their maximum KE depends on the frequency of the incident light (as long as this frequency is above threshold)
3. The pythons energy does work to relate the photoelectron and give it kinetic energy, the maximum KE of a photoelectron is the difference between the photons energy and the threshold energy
4. This is why the maximum kinetic energy of photoelectron increases with frequency

Question 7

What is a photoelectron?

Answer 7

An electron emitted during photo-electric effect

Question 8

What happens when a photoelectron is emitted?

Answer 8

1. Energy from a photon is used to realise a photoelectron and give the photoelectron kinetic energy
2. Conservation of energy means that energy from the photon equal the threshold energy plus the kinetic energy of the photoelectron

Question 9

What is the work function?

Answer 9

1. The work function a material is the least energy needed to release a photoelectron from a material and this is equal to the threshold frequency

Question 10

What is Einstein’s photoelectric equation?

Answer 10

hf = phi + 1/2mvmax^2

Question 11

What is the difference in maximum kinetic energy for the location of electrons?

Answer 11

• Electrons emitted from the materials surface have the maximum Kinetic energy because energy is NOT used moving to the surface
• Electrons from deeper in the material have less kinetic energy as some energy is used moving tot he surface

Question 12

What is a semiconductor?

Answer 12

• A material with conductivity between a metal and an insulator
• Treated to have low wf and respond to visible light and infrared radiation e.g. in digital camera

Question 13

How is the work function used in real life?

Answer 13

• Less than 3.1eV, visible light can release electrons

- Less than 1.77 eV, infrared light can release electrons

Question 14

What is the stopping potential?

Answer 14

• Gives a measure of the electron’s kinetic energy because the work done by the electric field eV is equal to the photoelectron’s kinetic energy
• So the Ke(max) = EVstop

Question 15

What is electron diffraction?

Answer 15

• Momentum of an electron: p=mc
• E = pc
• p = hf/c

Question 16

What is the de Broglie wavelength?

Answer 16

• For a particle is the Planck constant divided by a particles momentum and it represents the wavelength of a moving particle lamda = h / p
• Shows the FASTER moving particles have SHORTER wavelength

Question 17

How can diffraction be used with crystals?

Answer 17

1. Diffraction is greatest when the wavelength of a wave is roughly equal to the size of the gap it passes through
2. The wavelength of X-rays is of the same order as the magnitude as the spacing in many crystals
3. Wavelength of electrons similar to that of X rays so the same crystals should diffract electrons and X rays

Question 18

What is wave particle duality?

Answer 18

It is the ides that matter and radiation can be described best by sometimes using a wave model and sometimes using a particle model (the photoelectric effect can only be explained using particle model of EM radiation)