Photoelectric effect.

 Photoelectric effect;

The emission of electrons by a substance under the action of light is called Photoelectric effect.

Photoelectric Effect.

Experimental Setup:

The phenomenon of photoelectric effect can be studied with the help of an apparatus shown in Figure below. Within an evacuated glass jacket two electrodes R and S are enclosed and the light radiation is allowed to enter the jacket through a quartz window. The radiation falls on electrode R, called cathode. The electrode S can be kept at desired (positive or negative) potential with respect to the cathode. A sensitive ammeter is put in the circuit to record current resulting from photoelectrons. The potential difference between the cathode and anode can be measured by voltmeter.

Experimental Setup.

Experimental observations

The experimental observation of photoelectric effect may be summarised as follows;

1) Effect of Intensity of light on Photoelectric current;

For a constant potential difference between the cathode and anode, the number of electrons emitted from the cathode (and hence the photoelectric current) increases with increasing intensity of radiation i.e the photoelectric current is directly proportional to the intensity of light used.

Photoelectric current vs intensity of light.

2) Effect of potential on Photoelectric current;

For a fixed intensity and frequency of incident radiation the photoelectric current increases with increase in the potential applied to the anode and reaches a constant value (saturation current), beyond which further increase in potential does not affect the photoelectric current. On the other hand, if the plate S is made more and more negative with respect to the photocathode surface the current decreases. This negative potential (with respect to cathode) of the plate is called retarding potential. For a particular value of retarding potential, the photoelectric current becomes zero. This potential is called cut-off or stopping potential V₀ and is measure of maximum kinetic energy of photoelectron and thus the maximum kinetic energy (K.Eₘₐₓ) of the photoelectrons (with charge e) can be determined from the stopping potential V₀

K.Eₘₐₓ= eV₀

Potential vs Photoelectric current.

3) Effect of frequency of incident radiation on stopping potential;

We can see that for higher frequency values like 𝛎₃  the stopping potential is more negative or greater than the stopping potential for smaller frequencies like 𝛎₁. Thus the stopping potential is directly proportional to the frequency.

Effect of frequency on stopping potential.

4) For each substance there exist a characteristic frequency 𝛎₀ such that for each radiation with frequency below 𝛎₀, the photoelectrons are not ejected from the surface. This frequency is called the threshold frequency and the corresponding wavelength is called threshold wavelength.

5) The stopping potential and maximum kinetic energy of photoelectrons is independent of the intensity radiation and depends only on frequency of radiation.

6) There is no time lag between the incidence of radiation and ejection of electron.

Note: This phenomena was given by Hertz but was unable to explain many experimental observations of photoelectric effect which was later explained by Einstein.

Questions on Photoelectric Effect;

Q1) What is Photoelectric Effect?

Ans) The emission of electrons by a substance under the action of light is called Photoelectric effect.

Q2) What is stopping potential?

Ans) For a particular value of retarding potential, the photoelectric current becomes zero. This potential is called cut-off or stopping potential.

Q3) What is retarding potential?

Ans) The negative potential (with respect to cathode) of the plate is called retarding potential.

Q4) What is threshold frequency?

Ans) For each substance there exist a characteristic frequency 𝛎₀ such that for each radiation with frequency below 𝛎₀, the photoelectrons are not ejected from the surface. This frequency is called the threshold frequency