Photo-Electric Effect Experiment ~ Science Physics

Photo-Electric Effect Experiment

What is photo Electric Effect? What is work function?
Definition

When light of suitable frequency falls on metal surface, the electrons are emitted. These are called photo-electrons and process is known as “Photo electric effect”.

Photo-electric effect Experiment

Experimental observation

1) The number of photo-electrons (photo-current) is directly proportional to the intensity of incident radiation.

2) It is spontaneous emission. Emission of electrons occurs immediately after the radiation falls on the surface.

3) Electrons are emitted from the metal surface, no matter the incident radiation is weaker in intensity. Emission of electrons from metal surface is independent of intensity of the radiation.

4) Emission of electrons depends upon the threshold frequency, if frequency of incident radiation is higher than the threshold frequency then electrons will be emitted without decay.

Ø Above results cannot explained by the photo-electric effect. The classical theory cannot explain the threshold frequency of light.

Ø The results cannot be explained on the basis of electromagnetic wave theory of light.

Quantum mechanical Explanation

Einstein put forward Quantum mechanical explanation of the photo-electric effect using Planck’s black body radiation theory.

Theory

1) Incident radiation in photo-electric effect consist of tiny particles carry quanta of energy called photon like tiny oscillator defined in black body radiation. E=hѵ

2) On interaction of those photons with electrons of metal. All of the energy of photons is transferred to electrons.

3) The acquired energy from photons by electrons is used up as follows.

Overcome the work-function of metal

1) Releases the electron from metal surface.

2) Remaining energy is converted into the K.E of the electrons.

Mathematically

i.e. hѵ= hѵ₀+K.E

hѵ₀₌Work function ѵ₀₌ Threshold frequency

Measurement of work function

K.E of emitted electrons i-e Photo electrons can be measured by collecting electrons at anode i-e at negative potential. The voltage at which Electrons are stopped is called as stopping potential.

Mathematically.

K.E=eѵ₀Where ѵ₀= stopping potential

Ø hѵ = hѵ₀+ev

h(ѵ- ѵ₀) = K.E

If ѵ <ѵ₀Then K.E< 0

Which is not true. Hence no electrons can be emitted from the metal surface if incidents frequency is less than threshold frequency.