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photoelectric effect
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ABSTRACT
This study discusses the variations of photoelectric current with intensity of light. The
photoelectric effect is dependent upon various factors including frequency of light, intensity of
light, nature of material, energy of light and potential difference. However, even if the
photoelectric effect is caused, the photoelectric current which is produced as a result of it may
vary if intensity of light is changed, provided that frequency of illumination is greater than
threshold frequency. To determine the impact of changing light intensity on photoelectric effect
while keeping the other factors constant, an experiment was performed. The impact was seen by
changing the distance of light source from photocell and the recording the readings on
Microammeter. The correlation and graphical analysis was done to draw the results. The
correlation analysis showed a significant positive relationship between photoelectric current and
intensity of light. The graphical analysis also highlighted a positive linear relationship.
Therefore, it was concluded that intensity of light directly affects the variations in photoelectric
current.
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INTRODUCTION
The phenomenon of photoelectric effect was discovered by a German physicist Heinrich Rudolf Hertz in 1887. There are some factors which impact the process of photoelectric effect including frequency, energy and intensity of light. The variations in photoelectric current which is produced as a result of photoelectric effect are caused by the intensity of light. Various studies have been conducted to explain the relation between variations of photoelectric current and intensity of light.
The photoelectric current variations can be explained through Maxwell’s equations. According to Maxwell’s equations, the magnitude of electric field increases as the intensity of light increases because the magnitude of electric field vector of a light wave is directly proportional to square root of the intensity of light. The force of electron is positively related to the electric field vector which means that with the increase in the intensity of incident light, the kinetic energy of photoelectron should be increased (Javier 2008).^1
The Classical wave theory predicts that the phenomenon of electric effect is caused when the light of certain intensity and any frequency incidents on the surface of matter and it is intense enough to eject the photoelectrons. Although classical theory supports the relationship between variations in photoelectric current and intensity of light however, it is challengeable because in actual practice after a specific frequency photoelectric effect is not caused (or in other words photoelectric current is not produced) regardless of the intensity of light (Subhrajyoti).^2
Einstein made use of Plank’s theory to explain the photoelectric effect. According to his explanation, the light travels in the form of bundles of packets with an energy of E = hf* and when it falls on the surface of matter, it transfers energy into electrons and electrons come out of the surface of the matter and photoelectric current is produced. Einstein theory shows that intensity of light is positively related to the variations in photoelectric effect provided that light of specific frequency is being used.
(^1) Accessed from (^2) Accessed from http://www.wbabin.net/science/maji4.pdf
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In short, equations of Maxwell supports that intensity of light positively affects the variations in photoelectric current. The Classical theory also supports that intensity of light of any frequency positively affect the photoelectric current. However, quantum theory explains that only with a specific frequency of light, the intensity of light is directly proportional to photoelectric current. Therefore, in order to consolidate the results of these three different theories and to study the variations of photoelectric effect with the change in the intensity of light, the need to conduct this research study was felt.
This study starts with the theory section which briefly covers the process of photoelectric effect, factors affecting this phenomenon and applications of photoelectric current in our daily life. In the next section, I have explained the methodology that have been used to study the relationship between variations of photoelectric current and intensity of light. After that the experiment has been explained through which data has been collected. In the next section, the primary data collected has been summarized. In the last section, an analysis and evaluation has been done to interpret the data and findings. The study wraps up with a brief conclusion of the study and results.
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Uses of Photoelectric current
Photoelectric effect is a very useful phenomenon and its importance can be understood from following uses of the photoelectric current.
Basically the photoelectric current produced as the result of photoelectric effect is used in different types of photodiodes and phototransistors. The photoelectric current is very useful in many solar light sensitive diodes and solar power such as solar cells. In semiconductors, photoelectric current is produced by exciting the electrons or by kicking out electrons from the valence shells by throwing light even of low energy. This current produced in semiconductors is used for different purposes and has voltages related to the band gap energy.
One of the major uses of the photoelectric current is in photomultipliers. In Photomultipliers, the current is used for the detection of low levels of light. In the early days of television, the photoelectric current was also used in video camera tubes. The photoelectric current is produced in Silicon image sensors by knocking out the electrons from outer most shell of the solid but not out of the matter. This photoelectric current is then used in different charged couple devices. In addition, Photoelectric current is also used in gold leaf electroscope which is designed for the detection of electricity.
Photoelectron spectroscopy is also included in one of the main applications of the photoelectric current. The energy of incident photons can be found with the help of photoelectric current
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because energy of incident photons is equal to the sum of binding energy of material’s work function and the energy of photoelectrons. The binding energy is determined by bombarding the matter with ultraviolet source or monochromatic X-ray source. On the other hand, energy of emitted photoelectrons is determined by measuring the kinetic energy of photoelectric current.
Photoelectric current of positive or negative charges is also produced in space craft due to the photoelectric effect. The parts of space craft exposed to the shadow develop a negative current of several kilovolts. On the other hand, the parts of space craft exposed to light produce a positive current.
Furthermore, the sunlight hits the lunar dust, they get charged due to photoelectric effect. The surface of the moon is lifted off due the repulsion of this charged dust. So, the photoelectric effect is also used to study the surface of the moon. Apart from all of the above mentioned uses, photoelectric current produced during photoelectric effect also has a great use in night vision devices. The photoelectrons are ejected out when light is fallen on gallium arsenide plate of the night vision devices which are then amplified into cascade of electrons. These amplified electrons are used to lighten up a phosphor screen.
In short, there are various applications of photoelectric effect and current in our daily life.
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METHODOLOGY
This study is an experimental research study. It focuses on the variation of photoelectric current by changing the intensity of light. The quantitative experimental approach has been used to study the relationship between the two factors. Apart from that, findings of similar previous research studies and theories have been consulted to support the findings of this research study.
The experiment has been conducted in the laboratory environment to study the impact of independent variable on the dependent variable. It is one-point in time study and it is not a continuous study. Total nine readings have been taken by conducting the experiment. Since, it only studies the relationship between the variations of photoelectric current and intensity of light therefore, all other variables have been kept constant while carrying out the experiment.
After collecting the data by performing the experiment, it has been analyzed by two steps. In the first step, the correlation analysis has been done to explain the significant relationship between the two variables while in the second step; the graphical description has been used to interpret the findings.
Two variables have been selected to conduct the research. The dependent variable is the current whereas; the independent variable is the distance. Since, it has been investigated that by changing the distance, the magnitude of current changes therefore, current is the dependent variable.
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EXPERIMENT
To Study the Variation of Photoelectric Effect with Intensity of Light
Aim of Experiment: The aim of performing this experiment was to check the impact of
change in intensity of light on photoelectric current. Apart from that, it has also proved that the photoelectric phenomenon, which has been commonly used in various applications of daily life is a simple phenomenon.
Apparatus
The apparatus, which I have used to conduct this experiment, include Photocell, Rheostat, Electric Lamps, Sensitive Galvanometer/Microammeter, Battery, Scale, Key and Connecting wires.
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I = constant/d 2
I = 1/d 2
Since, constant = 1 (as there was no change in angle and relationship is linear)
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Precautions and Sources of Error
It was a very simple experiment however, in order to avoid the impact of other factors on the experiment, I took following precautions:
i. I took distance readings by placing the lamp at larger distance and then I changed the lamp with regular steps. The reason was that the smaller values of distance might have reduced the accuracy of readings. ii. The same light source was used for all readings to keep the frequency constant. However, before conducting the experiment, I ensured that no other electric source was attached to the apparatus and the light of right frequency was selected. Furthermore, all other lights of the laboratory were switched off before performing the experiment. iii. There are two kinds of photocells available in the laboratory including those which have external voltage source and those which do not have external voltage source. I performed the experiment by using the one without external voltage source. iv. To make my readings précised I used a Microammeter and my readings of current were measured in Microampere. Although a sensitive galvanometer could be also used however, a galvanometer is typically used to check the effect of photoelectric effect only and it does not give precise measurements of current. v. In order to avoid the inconsistency in data the distance of lamp from cathode was changed linearly without changing the angle at which light falls on cathode. vi. The photocell was not exposed to stray light because it could have distorted the aim of keeping the same frequency of light.
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PRIMARY DATA COLLECTED The primary data which has been collected after conducting this experiment is summarized in the give table. Table 1: Data Table Shows the Values of the Distance of Lamp from Photo Cell, Measured and the Deflection of Galvanometer.
Serial # Distance of lamp from photo cell
Deflection of Galvanometer
d^2 (±0.1)
1/d^2
'd' cm Microampere (I) cm^2 cm- 1 95.0 5 9025 0. 2 90.0 9 8100 0. 3 85.0 15 7225 0. 4 80.0^17 6400 0. 5 75.0 20 5625 0. 6 70.0 23 4900 0. 7 65.0 25 4225 0. 8 60.0 30 3600 0. 9 50.0 35 2500 0. 10 45.0 50 2025 0. 11 40.0 66 1600 0. 12 35.0^115 1225 0. 13 30.0 170 900 0. 14 27.0 216 729 0. 15 25.0 236 625 0. 16 23.0 252 529 0. 17 20.0^290 400 0. 18 15.0 339 225 0.
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The second column of the above table shows the distance of lamp from photocell. This distance is the distance of lamp from the cathode of photocell. Basically when the distance is increased, light from the sources takes more time to strike on the cathode and the intensity of the light is decreased. It means that to change the intensity of light, I have changed the distance of light source from the cathode of photocell.
The third column shows the deflections in galvanometer. Galvanometer shows the readings of current produced as a result of photoelectric effect. While conducting the experiment, the change in distance or intensity of light was also changing the deflections in galvanometer, thereby showing variations in photoelectric current.
The readings of fourth column have been calculated by taking the square of distance of lamp from cathode given in the second column. And finally to check the relationship between current and inverse of square of distance (I = constant/d 2 ), the readings in the last column were calculated.
In order to check whether there is a relation between photoelectric current and inverse of square of distance or not, correlation of two variables was calculated. The reading of correlation not only gives an idea about the nature of relationship between the two variables but it also provides a fair idea about the strength of relationship. The calculated value of correlation between photoelectric current and inverse of square of distance is 0.98738.
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Uncertainties
The uncertainty in this experiment has been collected by dividing the least count with calculated
observation. The least count of meter ruler scale is 0.1 cm and suppose, if the first observation is
95, then calculated uncertainty is 0.1/95=0.0011. The following chart shows the uncertainty for
each value.
Serial # Distance of lamp fromphoto cell (cm) Least count Uncertainties
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