8. The Photoelectric Effect

- If you shine radiation at a high enough frequency onto a surface it will emit electrons (this is usually within the UV range).

- If an electron absorbs enough energy from the radiation then it can break the bonds to the metal and be released.

Conlusions:

i) No photoelectrons are emitted fue to being below a threshold frequency

ii) Photoelectrons are emitted with varying KEs. The max KE increases with the frequency.

iii) The intensitiy is the energy per second hitting an area of metal. The max KE is unaffected by different intensities.

iv) The no. of photoelectrons emitted per sec is proportional to the intensitiy 

- Wave theory would say that it photoelectrons would be emitted after a certain time dependent on frequency but no matter what frequency. This is however wrong. Wave theory also can't explain why the KE depends only on the frequency, since it should be proportional to the intensity.

- Between Planck and Einstein's observations it was discovered that EM waves acted like particles in packets of photons and so therefore the photon model can be used to explain the photoelectric effect.

You can demonstrate the effect by this experiment:

- A zinc plate is attached to an electroscope

- The plate is negatively charged along with the box casuing the gold leaf to rise up.

- A UV light is then shone onto the plate, where electrons are lost.

- As the plate and box looses it's charge ot can be observed that the gold leaf falls down.

- When the EM radiation hits the metal surface if one of the photons collides with an electron it will gain the energy of 'hf'

- Before an electron can escape it needs a certain amount of energy determined by the metal, this is the work function. If the energy gained is more or equal it will be released, if not the electron will release the energy as a photon and the metal will simply heat up from the electron vibration.

- Since for an electron to be emitted hf >= the work function, then there has to exsist a threshold frequency, where f0 = work function/h

- The KE the electron carries when leaving the surface is the overall energy minus any other losses, therefore Ek = hf - work function.

- You can measure the…