Sunday, 25 May 2014

Photoelectric Effect

Photoelectric effect is one of the most fascinating yet simple phenomenon in physics. Heinrich Hertz carried out the experiment in which he took a tube and placed two electrodes in it. He then connected them to a power source. He coated the cathode with cesium metal as it has a low ionization energy, which means less energy is required to free the electrons. When sunlight was allowed to fall on the cathode, there was current in the circuit. The number of electrons the were freed depended on the intensity of the incident light whereas the energy required to free the electrons from the atom depended on the frequency of the incident radiation.

Photoelectric effect could not be explained by considering light in the form of a particle or a wave or electromagnetic radiation. In order to explain black body radiation Max Plank, a German theoretical physicist, considered light as a form of electromagnetic radiation composed of photons. This assumption helped him to explain black body radiation. For this he said that the energy of a photon is directly proportional to the frequency of radiation.
E = hv  where h is plank's constant and v is the frequency of the radiation.

Photoelectric effect had to be explained in such a way that it matched the observations of the experiment. Albert Einstein gave a perfect explanation to photoelectric effect which matched the observations and thus won him a noble prize. He called the minimum energy required to free the electrons must directly be proportional to the frequency of radiation as E = hv. He called this frequency as threshold frequency. Now that the electrons are free you need to give them more energy for them to gain kinetic energy and start moving towards the anode to constitute an electric current.
Kinetic Energy(max)  Kmax = h(ƒ − ƒ0)   where ƒ and ƒ0  represent the frequency of the radiation and h is the plank's constant.

Now how does intensity affect the number of electrons freed? Its simple, more intensity means more number of atoms will get the light hence more electrons will be freed. Hence the current, which is the flow of electrons through a particular point per second, is directly proportional to the intensity of the incident light.