4.5 Quantum Physics Flashcards
What are photons?
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
True or false: the energy of a photon is proportional to the wavelength of the light
False
It’s proportional to the frequency
E=hc
Energy is inversely proportional to the wavelength
What is the h in the equation E=hf
h is the Planck constant measured in Js
What quantity can be measured in electron volts?
Energy
Describe an experiment which can be used to estimate the Planck constant
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 the current flows in the circuit can then be used to find h from h=E/f
Describe how to improve the accuracy of the estimate of this experiment
Repeat with a variety of different coloured LEDs, which each emit different wavelengths of light
The values of wavelength and threshold pd for each can be recorded and a graph of V against 1/lambda
Gradient equals hc/e
Speed of light and electron charge are known constants, calculate value of h
What is the photoelectric effect?
A phenomenon where shining light with enough energy onto a metal releases electrons (and can cause a current to flow)
Electrons emitted are called photoelectrons
Which features of the photoelectric effect can’t be explained if light is a wave?
If light was a wave, then 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 spends on frequency (and no electrons are released below a certain threshold value, no matter how intense the light is)
How many photons does each photo electron absorb prior to emission?
Only 1
If I doesn’t contain enough energy the electron will re-emit the energy rather than being released
