Particles of Light

Question 1

What is the Photoelectric Effect

Answer 1

The idea that photoelectrons are emitted from the surface of a metal after the threshold frequency is reached (the threshold frequency varies for different materials)

Question 2

What is Threshold Frequency

Answer 2

The minimum amount of frequency needed for photoelectrons to be emitted from the surface of a metal

Question 3

Describe the effect of increasing intensity during the photoelectric effect

Answer 3

• By increasing the intensity of incident radiation more photons can do more work on the metal at the same speed and with the same kinetic energy
• So the ratio of 1 photon to 1 electron would increase so more more electrons would be emitted
• So the current would increase

Question 4

What is the effect of increasing the number of photons and changing the frequency of the light during the photoelectric effect

Answer 4

• More photons would increase the intensity of light but the photons would have the same individual kinetic energy
• More frequency would increase the kinetic energy of each photon but the intensity of light would be the same

Question 5

What is PhotoElectricity Equation

Answer 5

hf = ɸ + Ekmax

Planck’s Constant (J s ) x Frequency (Hz) = Work Function (J) + Max Kinetic Energy(J)

Question 6

Explain why the kinetic energy of an emitted electrons during the photoelectric effect has a maximum value

Answer 6

• The energy received will be the same energy that the photon has
• The energy required to remove the electron varies so different electrons have different kinetic energies

Question 7

What is the Work Function

Answer 7

The minimum amount of energy needed to free an electron from a metal surface

Question 8

Describe Graph of Maximum Kinetic Energy of a Photoelectron

Answer 8

Question 9

What happens to the Maximum Kinetic Energy Graph if the Work Function changes

Answer 9

The line will be parallel because the gradient is a constant

Question 10

Explain Stopping Potential

Answer 10

• By making the potential of the right hand electrode negative
• The photoelectrons can be turned back so they do not reach the right hand electron
• At this point current = 0
• We say we have applied a stopping potential
• The stopping potential = photoelectron’s kinetic energy

Question 11

What is Stopping Potential

Answer 11

The potential difference needed to apply across a metal to stop the photoelectrons with the maximum kinetic energy

Question 12

What is stopping potential equal to

Answer 12

Max Kinetic Energy / 1 electron volt (eV)

Question 13

Explain why in the photoelectric effect eventually the currect produced reaches a constant value

Answer 13

A constant current is reached when all the electrons have been emitted

Question 14

Describe the Principal of Wave-Particle Duality

Answer 14

It is sometimes helpful to think of waves as behaving like particles and also particles behaving like waves

Question 15

Explain why the photoelectric effect can not be explained by the wave theory

Answer 15

• Because wave theory suggests that any frequency of light should be able to cause photoelectric emission since the energy absorbed by each electron will gradually increase with each incoming wave, but this is not true
• There is no photoelectric emisson below the threshold frequency
• If we model the photoelectric effect as particles then we can say electromagnetic waves travel in discrete packets called photons, which have an energy which is directly proportional to frequency
• Each electron can absorb a single photon, therefore a photoelectron is only emitted if the frequency is above the threshold frequency

Question 16

What is the De Broglie Wavelength Equation

Answer 16

⅄ = h/p = h/mv

Wavelength = Planck’s Constant / Momentum = Planck’s Constant/ mass x Velocity

Question 17

What are Momentum Units

Answer 17

Kg m s-1 or ns

Question 18

What are the Properties of Waves and Particles

Answer 18

Properties of Waves-

• Interference
• Diffraction
• Refraction

Properties of Particles -

• Detected in discrete units
• Photoelectric Effect

Question 19

Why do existing scientific theories need to be modified or replaced

Answer 19

Because a new theory makes predictions that are tested by repeatable experiments or new evidence is obtained

Question 20

Suggest a suitable material to give an observable diffraction pattern with electrons

Answer 20

Crystal or Graphite since atomic spacing ≈ λ electrons