Skip to main content

Photoelectric effect.

 Photoelectric effect;

The emission of electrons by a substance under the action of light is called Photoelectric effect.
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 for the study of photoelectric effect.
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.
Effect of Intensity of light on Photoelectric current.
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
Effect of Potential on Photoelectric current.
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 of radiation on stopping potential.
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

Comments

Post a Comment

Popular posts from this blog

Mass defect, packing fraction and binding energy.

 Mass defect, packing fraction and binding energy: It was assumed that mass of the nucleus is equal to the mass of its constituents (i.e protons and neutrons). But experimentally it was found that the actual mass of the nucleus is less than the theoretical mass. Thus, the difference between the theoretical mass and experimental mass is called mass defect i.e ∆m={[Zmₚ + (A-Z)mₙ] - M} Where mₚ= mass of proton              mₙ= mass of neutron               M= actual mass of nucleus                Z= atomic number                A= mass number The ratio of mass defect and mass number (A) is called packing fraction (f) f = ∆m/A Thus packing fraction is the mass defect available per nucleon. The packing fraction explains the stability of the nucleus. The packing fraction may be positive, negative or zero. The positive value of packing fraction indicates that the nuclei is unstable while the negative value of packing fraction indicates that a fraction of nucleus mass has been converted into e
Post a Comment
Read more

Different kinds of beta decay.

 Different kinds of beta decay 1) Negative beta decay process: When there is excess number of neutrons in the nucleus, the neutron is converted into proton with the emission of electron and antineutrino particle and this process is called negative beta decay process. Negative beta decay. 2) Positive beta decay process: When there is excess number of protons in the nucleus, the proton is converted into neutron with the emission of positron and neutrino particle and this process is called positive beta decay process. Positive beta decay. 3) Electron Capture: When there is excess number of protons in the nucleus, sometimes the nucleus will absorbed the nearby electrons in the nearest electron orbital emitting neutron and a neutrino and this process is called electron capture. Electron capture. 4) Inverse beta decay: Inverse beta decay. Thus such kind of reaction in which neutrinos are absorbed to create some sort of beta decay is called inverse beta decay. Inverse beta decay confirm the e
Post a Comment
Read more

Beta decay

 Beta decay; Beta decay is a spontaneous radioactive decay process in which either a neutron gets converted into proton or a proton gets converted into neutron with the emission of electron or positron respectively. Whenever proton or neutron are in higher energy level because they are in excess they get converted into another kind of particle and thus decreasing the energy of the system and increasing its stability. In ¹²B there are seven neutrons and five protons while in ¹²C there are six neutrons and six protons, since both protons and neutrons are fermions and no more than two same kind of fermions can occupy the given energy level. Thus the figure below shows the nuclear energy diagram of ¹²C and ¹²B. Since in ¹²B nucleus, there are two neutrons present in excess, if a neutron gets converted in proton, then this new proton occupies an energy level lesser in energy than that of neutron and thus giving a stable ¹²C nucleus. Pauli theory of beta decay; An interesting historical deve
Post a Comment
Read more