Module 4.5

Question 1

What is Photons?

Answer 1

Photons are particles of light.

Question 2

What quantity can be measured in electron volts (eV)?

Answer 2

Energy

Question 3

Describe an experiment which can be used to estimate the value of the Planck constant.

Answer 3

• An LED will only allow current to pass after a minimum voltage has been put across it.
• At this voltage all the electrons will have the same energy as a photon emitted by the LED (which you will know the frequency of).
• Finding the threshold voltage by seeing when current flows in the circuit can then be used to find h from: h = E/f

Question 4

What is photoelectric effect?

Answer 4

The photoelectric effect is a phenomenon where shining light with enough energy onto a metal released electrons (and can cause a current to flow). The electrons emitted are called photoelectrons.

Question 5

Which features of the photoelectric effect cannot be explained if light is a wave?

Answer 5

If light was a wave then the energy of the electrons released would increase with increasing intensity of the light - but this is not the case. Instead the energy of the electrons depends on frequency (and no electrons are released below a certain threshold value, no matter how intense the light is)

Question 6

How many photons does each photoelectron absorb prior to emission?

Answer 6

Only 1. If it does not contain enough energy the electron will re-emit the energy rather than being released.

Question 7

How does the photon model of light explain the threshold frequency seen in the photoelectric effect?

Answer 7

Each electron absorbs a single photon. This single photon must have enough energy for the electron to be released if it does not the energy is re-emitted. The electron cannot build up energy as it could if light was a wave.

Question 8

What is the name given to the minimum amount of energy an electron required to leave the surface of a metal?

Answer 8

The work function (or ‘work function energy’), Φ

Question 9

Write a word equation relating the energy of an incident photon to the work function and the kinetic energy of released electrons.

Answer 9

Photon energy = work function + kinetic energy.

Question 10

What experimental evidence appears to show particles behaving as waves?

Answer 10

• Electron diffraction
• Electrons will diffract is passed through the spaces between atoms in graphite (like a tiny diffraction grating).
• This would not happen if electrons were behaving as particles only.

Question 11

Which equation relates the wave and particle properties of electrons

Answer 11

The de Broglie equation:
λ = h/p

where λ = wavelength (wave-property), h = Planck’s constant and p = momentum (particle property)