quantum physics Flashcards
describe experiment which can be used to estimate the value of the Planck constant
- 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 LES which you know the frequency of
- finding the threshold voltage by seeing when current flows in the circuit can then be used to find h = e/f
describe how to improve the accuracy of the estimate of this experiment
- to improve accuracy of experiment the experiment can be repeated with a variety of different coloured LEDs which each emit different wavelength of light
- the values of wavelength and threshold pd for each can be recorded and a graph drawn of v against 1/lambda
- the gradient of this graph will be equal to hc/e
- as the speed of light and the electron charge are known constants can calc h from this
photoelectric effect
phenomenon where shining light with enough energy onto a metal releases electrons and can cause current to flow. the 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 the energy of the electrons released would increase with increasing intensity if the light - but this isn’t the case. instead the energy of electrons depends on frequency and no electrons are released below a certain threshold value no matter how intense the light is
how does the photon model of light explain the threshold frequency seen in the photoelectric effect
each electron absorbs a single photon. this single photon must have enough energy for the electron to be released, it it doesn’t the energy is re-emitted. the electron can’t build up energy as It could if light was a wave
true or false: the rate of emission of photoelectrons is proportional to intensity
true
higher intensity means more photons, this means more electrons can absorb energy and be released
does the maximum ke of a released electron depend on the intensity of light hitting the surface
no, energy transferred is due to a one-to-one interaction and so depends on frequency not intensity
what experimental evidence appears to show particles behaving as waves
- electron diffraction
- electrons will diffract is passed through the spaces between atoms in graphite
- this wouldn’t happen if electrons were behaving as particles only