Laws of Photoelectric Emission

1. For a light of any given frequency, photoelectric current is directly proportional to the intensity of light, provided the frequency is above the threshold frequency.

2. For a given material, there is a certain minimum (energy) frequency, called threshold frequency, below which the emission of photoelectrons stops completely, no matter how high is the intensity of incident light.

3. The maximum kinetic energy of the photoelectrons is found to increase with increase in the frequency of incident light, provided the frequency exceeds the threshold limit. The maximum kinetic energy is independent of the intensity of light.

4. The photo-emission is an instantaneous process. After the radiation strikes the metal surface, it just takes 10–9 s for the ejection of photoelectrons.

Source of Radiation

If P (in Watts) be the power of the source of radiation and I be the intensity of light radiation at a distance R from the source (R is the perpendicular distance of the receiving surface from the source), then: 

I = P/4πR2

If radiation is falling on a plate of area A, then, energy absorbed/sec by the plate is given as:

Energy / sec = I × A = (P / 4πR2 × A ) J/sec

If v be the frequency of the source (i.e. light radiation), then energy per photon = hv 

=> number of photons failing on plater per second = energy per second / energy per photon.

=> n/sec = P / 4πR2 × A / hv             (Energy / photon = hv = hc/λ)

If v > v0 (v0: threshold frequency of plate) and photon efficiency of plate be x %, then the number of photoelectron emitted per second is:

=> n/sec (P/4πR2 × A / hv) ×  x/100

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