Photoelectric Effect
The Photoelectric Effect demonstrates that waves can sometimes behave as particles, in Quantum Physics. This is what Einstein won his Nobel Prize for.
A photon is a ‘quantum of EM radiation’
It involves exposing a piece of metal to electromagnetic radiation. If the energy of the photons in the radiation is high enough, electrons will be given off from the metal. As electrons (Negative charge) are being given off, the resulting metal will have a positive charge.
E = h f
E = [Phi] + 1/2 m v[max]^2
P = E n
P = Power
n = Number of photons
E = Energy of photon
h = Planck’s Constant [6.63x10^-34]
f = Frequency of electromagnetic wave
[Phi] = Work function, the minimum energy required to emit electrons.
1/2 m v[max]^2 = The maximum kinetic energy that the photoelectron has upon leaving the metal.
m = mass
v = velocity
The threshold frequency is the frequency when:-
hf = [phi] (ie. the photoelectrons only just leave the metal). If hf < [phi], no photoelectric effect is observed.
1 electron Volt (eV) = 1.6×10^-19 J. Work function is normally measured in eV, as it makes numbers easier to manage.
The intensity of electromagnetic radiation affects the NUMBER OF PHOTONS that hit the metal, thus the number of electrons emitted
The frequency of electromagnetic radiation affects the ENERGY OF PHOTONS that hit the metal, thus the maximum kinetic energy of electrons emitted.
Electromagnetic waves can behave as particles or as waves.
Waves:-
- Shows diffraction
- Diffracted by slits
- Diffraction noticable when size of gap ~ [lamda]
- Shows interferences
- Shows polarisation
- v = f [lamda]
Particles:-
- Interaction of EM waves with matter
- Energy of photons given by E = hf
- E = energy of photon, f = frequency, h = 6.63×10^-34
- Photoelectric Effect
- One-to-one interaction between photon and electron
- Energy is conserved (hf = [Phi] + 0.5mv^2)
- Frequency affects the energy of photons
