Quantum Physics

Question 1

What are photons?

Answer 1

Photons are particles of light.

They are sometimes referred to as a quantum of energy of EM radiation. ‘A quantum’ in this context just means a set (finite) amount.

Question 2

True or False: The energy of a photon is proportional to the wavelength of the
light.

Answer 2

False.

It’s proportional to the frequency: E = hf

Energy is inversely proportional to the wavelength.

Question 3

What is ‘h’ in the equation E = hf? Give units

Answer 3

h is the Planck constant, measured in Js.

Question 4

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

Answer 4

Energy

Question 5

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

Answer 5

● 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 6

What is the photoelectric effect?

Answer 6

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

Question 7

Which features of the photoelectric effect can’t be explained if light is a wave?