The photoelectric effect is the name given to the phenomenon of emission of electrons from a metal surface when the light of a suitable frequency is incident on it. What happens when light falls on a metal surface? How can we study it? Let’s find out.
Experimental Study of Photoelectric Effect
The aim of the experiment is to study the emission of electrons by light. We also try and measure the energy of the electrons emitted in the process. In addition to this, we will also observe the relation of these electrons with the frequency of light used. To study the effect, we use an evacuated cathode ray tube connected in a circuit as shown below:
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Near one of the plates inside the evacuated tube, there is present a small quartz window. The Quartz window has two functions – it lets light in and it only lets the Ultra Violet light in. Hence by using a Quartz window, we make sure that light of a specific frequency falls on the metal plate inside the evacuated chamber.
We connect a voltmeter across the two plates. This measures the potential difference between the plates. Moreover, we have a sensitive galvanometer in the circuit. This measures the photocurrent.
The Collector plate-C emits electrons which are then collected at the collector plate-A. These plates are connected to the battery via the commutator. Let’s switch on this experiment and see what happens!
In The Beginning!
Well, in the beginning, let us have a zero potential. We open the quartz window and observe the reading of the Voltmeter and the Ammeter. Both will give a non-zero reading (say for alkali metals), proving the occurrence of the Photoelectric effect. As we increase the Voltage and change it again, we will make the following observations:
The Effect of Intensity
The number of electrons emitted per second is observed to be directly proportional to the intensity of light. “Ok, so light is a wave and has energy. It takes electrons out of a metal, what is so special about that!” First of all, when the intensity of light is increased, we should see an increase in the photocurrent (number of photoelectrons emitted). Right?
As we see, this only happens above a specific value of frequency, known as the threshold frequency. Below this threshold frequency, the intensity of light has no effect on the photocurrent! In fact, there is no photocurrent at all, howsoever high the intensity of light is.
The graph between the photoelectric current and the intensity of light is a straight line when the frequency of light used is above a specific minimum threshold value.
The Effect of The Potential
Suppose you connect C to a positive terminal and A to a negative terminal. What do you expect will happen to the photocurrent?
Since electrons are negatively charged, if we increase the negative potential at C, more and more electrons will want to escape this region and run to the attractive plate A. So the current should increase. Similarly, if we decrease the negative potential at C, removing electrons will become difficult and the photocurrent will decrease. Hence the maximum current flowing at a given intensity of incoming light is the saturation current.
As you can see in the graph, the value of saturation current is greater for higher intensities, provided the frequency is above the threshold frequency. Imagine you are an electron and you just escaped the metal surface. Now you are merrily accelerating towards A. What if we became mischievous and increased the negative potential at A? You will feel a repulsion and consequently you will lose speed.
What if the potential is very strong? You will not be able to escape the metal surface at all! As a result, we call this value of the potential for which the photocurrent becomes zero as the stopping potential or the retarding potential. The more the negative potential of the collector plate, the more is the effort that an electron has to make if it wants to escape successfully from the metal surface.
Thus we will get the following relationship between the stopping potential and the photocurrent.
Effect of Frequency
We see that for higher frequency values like ν3, stopping potential is more negative or greater than the stopping potential for smaller frequencies like ν1. What does this mean? This means that there should be a relationship between the frequency and energy.
In the End
We can sum up the observations as follows:
- For a given metal (photosensitive material), the photoelectric current is directly proportional to the intensity of the light used, above a minimum value of frequency called the threshold frequency.
- The saturation current depends on the intensity for a known value of frequency. At the same time, we see that the stopping potential does not depend on the intensity over a specific value of frequency.
- The Photoelectric effect does not occur below a certain frequency. This is the threshold frequency. If the frequency of light is above the threshold frequency, the stopping potential is directly proportional to the frequency. In other words, to stop an electron emitted by a higher frequency, we require more energy. The stopping potential provides this energy.
- All of this happens instantaneously. As soon as we open the quartz window, electron emission starts.
All this is beautifully explained by the Einstein’s Photoelectric equation.
Solved Examples For You
On reducing the wavelength of light incident on a metal, the velocity of emitted photoelectrons will become
A) Zero B) Less
C) More D) Remains Unchanged
Solution: C) It will become more.
We know that the energy of photoelectrons increases as we increase the frequency. This means that their kinetic energy will be more. Hence higher frequency means a greater speed of a photoelectron. We also know that λ = c/ν. Hence if the wavelength is increased, the frequency will be decreased and vice-versa. So lesser wavelength means greater frequency and greater speed of the photoelectrons.