- In today’s article, we are going to study about ‘Facts about photoelectric effect you must know to score high’. And How beneficial it is and what is actually photoelectric effect are. We will also read about some more topics like Hertz observations, experiment study of photoelectric effect, effect of intensity of light on photoelectric effect, effect of potential on photoelectric effect, effect of frequency of incident light on photoelectric effect and summary of photoelectric effect, etc in Detail. So, take your notebooks in you hand and get ready to study physics in an easy and sorted way.
Photoelectric effect contains many facts. Some of them is mentioned below-
1. Hertz’s observation
Photoelectric effect was discovered in 1887 by Hertz’s experiment of spark (electromagnetic wave generator) experiment. And He observed that if Cathode surface illumination by the ultra violet light then spark intensity has increased. Hence He did not carry this experiment further.
Experiment study of photoelectric effect
An experiment settup is given above diagram. It consists a vacuum glass tube has contain two plate one of them is photosensitive plate (c) and another a collector plate (simple metallic plate). We can apply potential between these plate using a battery, these plate are connected to battery through a commutator and a variable resistor. A micro ammeter and voltmeter is connected to measure photo electric current and voltage respectively. A commutator is use for changing voltage polarity between these plate.
1) Effect of intensity of light on photoelectric current
If we apply a ‘c’ potential between these plate ( plate A is positive with respect to plate c). If we take a suitable frequency of light and change its intensity. And observe the photoelectric current in the circuit then we found the photoelectric current is linearly increases with the intensity of light. In this experiment frequency of light remains constant. A graph between intensity of light and photoelectric current as shown in fig.
2) Effect of potential on photoelectric current
- By keeping the intensity and frequency of incident light constant, we study the variation of current with potential difference between E and c. The portion of graph towards the right of the current (i) axis shows that as the collector c is made even slightly positive.
- Almost all the electron ejected by the emitter (E) are collected by c. And soon the current attains its maximum value.
- The maximum value of the photoelectric current is called the saturation current. The portion of the graph towards left of the current axis shows the effect of making c negative with respect to E.
- Since the current does not immediately drop zero, it is obvious that the photoelectrons are emitted with velocities ranging from zero to a certain maximum.
- We find that Vo is that sharply defined negative potential which when given to ‘c’ stops even fastest electrons so that the photoelectric current reduces to zero. Whereas positive v given to ‘c’ is called the accelerating potential. The negative potential given to ‘c’ is called regarding potential.
- The retarding potential (given to the collector) for which the photoelectric current becomes zero is called the cut-off or stopping potential for the corresponding frequency of the incident light.
If m is the mass of electron and Umax is the maximum velocity of the most energetic ejected electron 1/2 m Vmax^2 = eVo …(1)
If Kmax is the corresponding kinetic energy
Kmax = (1/2)m Vmax^2 ….(2)
Thus, from equation (1) and (2)
Kmax = eVo
3) Effect of frequency of incident light on stopping point
If a fix intensity of light incident on photosensitive plate. And hence, If we increase the frequency of light from 0 to maximum value then we found the photoelectric current is zero. This is only for a certain frequency value below.
The stopping potential is also zero for a particular frequency of light. And The value of this particular frequency from this value of frequency, the photoelectric current will start and this frequency is known as threshold frequency.
Now we plot photoelectric current v/s stopping potential graph for different frequency values. This graph look like as given in figure
In this experiment, we have got some reason that are given below-
- The stopping potential depends upon the frequency of light. When frequency of light increase the stoping point value also increase. It is found that stopping point is directly proportional to the frequency of light.
- The saturation current does not depend upon the frequency of light.
- If we plot a graph between stopping potential and frequency for different material.
From this graph we found the stopping point is linearly increases with the frequency of light and we get a minimum frequency value there stopping value is ‘0’. This value is known as threshold frequency. We found that the threshold frequency value depend upon the material type.
Summary of photoelectric experiment
- Photoelectric current is directly proportional to the intensity of light and it doesn’t depends on the value of frequency.
- The stopping frequency value is depends upon the frequency of light and not depend on the intensity of light.
- Below of threshold frequency there is no current observe in the circuit and stopping potential value is also ‘0’ for this frequency.
- Their is no time lag between incident light and photoelectric current, hence when light incident on photosensitive surface immediately photoelectric and consequently current flow in the circuit. Photoelectric current takes 10^-19 sec time to start from the time of incident light.
So our article is finished and after completely reading this article, one can easily tell what is photoeletric effect. And how we can explain it , what is stopping potential, threshold frequency etc.
So one can say that they got a detailed information about photoelectric effect and basic information about Hertz observations, experiment study of photoelectric effect, effect of intensity of light on photoelectric effect, effect of potential on photoelectric effect, effect of frequency of incident light on photoelectric effect and summary of photoelectric effect, etc
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