Photo Electric effect
What photo electric effect?. It can be defined as the emission of electron from metal surface when photon or electromagnetic radiation fall on it shows on it. For some surface like sodium, the wavelength photons have to be short (ultra-violent or x-ray). To ensure emission. While for other such as cesium electrons were emitted when photon of long wavelength (infra-red radiation) fall on it. The photo electric effect was first observed by Hertz in 1887.
Conditions for photo electric effect to occur
- The number of photo electrons emitted is directly proportional to the intensity of the radiation
- The energy of emitted electrons depend on the frequency of the incident radiation
- There exists the threshold frequency for all surface , and below the frequency no electron is emitted , no matter the intensity of the radiation, so as long as the radiation with frequency above the threshold value is used the emission is instantaneous
- No electron is emitted from the positive plate, due to mutual attractive force between electrons and the plate.
Demonstration of photo electric effect.
Electrons emitted from the cathode are pushed toward the Anode by electric field force. If the direction of electric field is reversed, some electrons will still reach the anode. The kinetic energy of emitted electron at anode is zero, and have a potential energy of eVo ( Stopping potential).
K.Emax= ½ mv2max =eVo where evo = penitential energy (Stopping potential) at anode
Photo electric effect and quantum theory
To best explain the Photo electric effect, we resort to quantum theory as proposed by Max Plank, that the radiation is emitted in quanta, or packets of energy, which is given as
E =hf, where h = Plank’s constant (h=6 .62 x 10-34Js), and f = the frequency of the radiation.
It is only quanta of sufficient energy and frequency will cause Photo electric emission from metal surface. The minimum energy required to remove electron from metal surface is give as
Eo = hfo therefore K.Emax =½mv2max =hf – ϕ so
eVo =hf-ϕ where ϕ is known as work function
Einstein’s Photo electric equation
It is given as E= hf = ϕ + ½mv2 if ϕ =hfo where fo is the threshold frequency, then ½mv2 = h(f-fo) and evo = h(f-fo)