. Explain the experimental setup of Photoelectric Effect


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The apparatus consists of two photo sensitive surfaces A and B enclosed in an evacuated quartz bulb. The plate A is connected to –ve terminal of the potential divider while plate B is connected to the positive terminal through a galvanometer G or a micro ammeter. 

In the absence of any light there is no flow of current and hence there is no indication in the galvanometer or micro-ammeter, but when a mono chromatic light is allowed to fall on the plate A, a current starts / flowing in the circuit which is shown by galvanometer. 

The significant features of this experiment are : 

i. No photo emission (the emission of electrons from a metal after it has absorbed light energy) is observed to occur below a critical cut-off frequency vTH (and the corresponding cutoff wavelength TH) of the incident light. 

ii. When a photo material is illuminated by a light with v > vTH, the electron emission begins instantaneously with on observable time delay, even if the light intensity is very low. 

iii. For incident light with frequency v > vTH the number of electrons liberated is directly proportional to the light intensity. 

iv. For positive anode voltages, the electric field established in the tube effectively pulls all the liberated electrons to the positive terminal. If we increase the source potential difference V in the circuit of figure, no increase in the current occurs, because all the emitted electrons reach the anode. 

v. If we reverse the polarity of the potential difference, the photo current decreases. The direction of electric field has reversed, it prevents some of the electrons from reaching the anode. Only the electrons with kinetic energy greater than e(V) can make it to the negative plate and contribute to the current. When this reversed electric field is large enough, no electrons are collected and the current is zero. This observation proves that the emitted charge is negative and has range of kinetic energies from essentially zero up to a maximum value KEmax. 

vi. At some voltage VS¬, called stopping potential, no current will flow between the electrodes. The kinetic energy of electron is converted into electric potential energy as the electrons approach the negatively charged anode. At stopping potential the kinetic energy of even the fastest electron is completely converted into electric potential energy. 

KEmax = eVS¬ 

 

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