Friday, January 30, 2015

Photo Electric Effect Experimental Observations

When a light of suitable frequency incident on a metal surface, electrons are emitted from the metal surface and these electrons are called photo electrons. The corresponding current is called photo electric current and the phenomena is called photo electric effect.

Photo electric effect is possible with any of the metal surface when the light of suitable frequency is allowed to incident on the metal surface. The incident frequency shall have a minimum value for this photo electric effect to happen and the minimum frequency is called the threshold frequency. When the incident frequency is more than threshold frequency, photo electric effect can happen.

We can express the threshold value even in terms of wavelength. Being frequency is reciprocal to wave length; threshold wavelength is the maximum wavelength of the light that is allowed to incident on a metal surface therefore photo electrons can be emitted. It means when the incident light is having a wavelength less than the threshold wavelength, photo electric effect is possible.

To observe the properties of photo electric effect , experimental arrangement is made as shown below. The apparatus consists of a discharge due with the cathode and anode. Light is allowed to incident on the cathode. The anode is further connected to a rheostat and then further to input a voltage.

When the incident frequency is more than threshold frequency, from the cathode photo electrons are emitted and the emitted photo electric current is measured with the ammeter connected in the circuit.

It is noticed that the photo electric effect is instantaneous process. It means immediately after the striking of light, photo electrons are emitted. There is no time lag in between .

When the voltage is not applied, the photo electrons are not having enough energy to continue travelling in the circuit and to make a consistent current. The applied voltage is enabling the flow of the current through the circuit.

When no voltage is applied, the released electrons get struck between the cathode and anode and they are called stacked electrons. These electrons further oppose the flow of the current and to overcome it, we need to apply the voltage. With the applied voltage, we can notice a steady flow of current in the circuit.

It is experimentally observed that, with the increase of intensity of light, the corresponding photo electric current is also increasing. The graph drawn between intensity of the light in the photo electric current is a straight line passing through the origin.

When the positive plate of the battery is connected to the anode and the negative plate is connected to cathode, there is an increase in the photo electric current. If reverse voltage is applied to the cathode, that is connecting a positive plate to the cathode, it is practically noticed that with the increase of voltage, photo electric current starts decreasing.

At a particular reverse voltage, photo electric current becomes zero and this particular voltage is called stopping potential. At the stopping potential the kinetic energy of the electrons is compensated by the potential energy acquired by the electron due to the stopping potential. We can equate both the energies basing on the law conservation of energy.

It is also practically noticed that stopping potential is independent of intensity of light. With different intensity of light, there may be different photo electric currents. But for all the intensities, stopping potentially is same. It is represented on the negative x-axis of the graph. On this graph voltage is taken on x-axis and the photo electric current is taken and y-axis.

It is also practically noticed that, stopping potentially is a dependent of frequency of the incident light. It is noticeable that for different frequencies of incident light, the corresponding stopping potentially is different. It is also experimentally observed that change of the frequency of the incident light is not going to affect the saturation current that is generated.

It is experimentally observed that higher the incident frequency, more the stopping potential.

We can draw a graph taking the incident frequency on x-axis and the stopping potential on y-axis. The graph is as shown below. It is observed that the incident frequency shall be more than threshold frequency for the photo electric current to emit. Then only we can apply reverse voltage so that somewhere in the photo electric current stops. Once if the applied frequency is more than the threshold frequency, it is observed that with the increase of frequency, the stopping potential also increases.

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