Topic 4.5: Quantum physics

Question 1

what are photons ?

Answer 1

photons are particles of light
sometimes referred as a quantum of EM radiation

Question 2

describe an experiment which can be used to estimate the Planck constant

Answer 2

an LED will only allow current to pass after a maximum voltage has been put across it
at this voltage all of the electrons will have the same energy as a photon emitted by the LED
finding the threshold voltage by seeing when current flows in the circuit can the be used to find h from h = e/f

Question 3

what is the photelectric effect ?

Answer 3

shining light with enough energy onto a metal releases energy (and can cause a current to flow)
the electrons emitted are called photoelectrons

Question 4

which features of the photoelectric effect cant be explained if light is a wave ?

Answer 4

if light was a wave, then the energy of the electrons released would increase with increasing intensity of the light - but this isn’t the case .
instead the energy of the electrons depend on frequency (and no electrons are released below a certain threshold value , no matter how intense the light is)

Question 5

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

Answer 5

each electron absorbs a single photon. this single photon must have enough energy for the electron to be released , if it doesn’t the energy is re-emitted.
the electron cant build up energy as it could if light was a wave

Question 6

what is the work function ?

Answer 6

the minimum amount of energy an electron requires to leave the surface of a metal