Description

Photoelectric Effect is a well knownprincipleof Quantum Theory, taught in the higher grades insciencecurriculum across the globe. Particle Nature ofElectromagneticRadiation is one of the fundamental principle onwhich QuantumTheory is based. This is lucidly explained byPhotoelectric Effect,formulated by Einstein. The rules behind thiseffect can besummarized as:
1. No electrons are ejected, regardless of the intensity oftheradiation, unless its frequency exceeds a thresholdvaluecharacteristic of the metal.
2. The kinetic energy of the ejected electrons increaseslinearlywith the frequency of the incident radiation but isindependent ofthe intensity of the radiation.
3. Even at low light intensities, electrons are ejectedimmediatelyif the frequency is above the threshold.

These observations suggest that the photoelectric effect dependsonthe ejection of an electron when it is undergoing a collisionwitha particle-like projectile (the radiation) that carriesenoughenergy to eject the electron from the metal. If we supposethat theprojectile is a photon of energy hv, where v is thefrequency ofthe radiation, then the conservation of energy requiresthat thekinetic energy of the ejected electron should obey

hν = φ + Kinetic Energy
hν = φ + qVs where Vs is stopping potential.
Here φ is the work function which is Planck’s constant timesthethreshold frequency (v0), a frequency below which theelectronscannot be ejected. Therefore,
φ = hv0
This threshold frequency is the characteristic of the metal.
Important Results:
Increasing the frequency of the incident beam, keeping the numberofincident photons fixed (this would result in aproportionateincrease in energy) increases the maximum kineticenergy of thephotoelectrons emitted. Thus,the stopping voltageincreases.
The number of electrons ejected also changes because theprobabilitythat each photon results in an emitted electron is afunction ofphoton energy.
If the intensity of the incident radiation is increased, there isnoeffect on the kinetic energies of the photoelectrons.
Increase in intensity of incident beam (keeping the frequencyfixed)increases the magnitude of the photoelectric current,thoughstopping voltage remains the same.
Above the threshold frequency, the maximum kinetic energy oftheemitted photoelectron depends on the frequency of theincidentlight, but is independent of the intensity of the incidentlight solong as the latter is not too high.