Photoelectric Effect Experiment

The Phenomenon was discovered by Hertz. Philipp Lenard, Hertz’s student, conducted further experiments and observed the phenomenon by showing that light could eject electrons from a metal surface. J.J Thomson identified the ejected particles as electrons, and later, Robert Millikan performed further experiments to further solidify the understanding of the photoelectric effect by studying the relationship between light frequency and electron kinetic energyEnergy is the ability to do work. Energy exists in several forms such as heat, kinetic or mechanical energy, light, potential energy, and electrical energy. Units of Energy: The SI unit... More.

Photoelectric Effect Experiment:

The phenomenon is basically about converting light energy to electrical energy.

The apparatus consists of two electrodes in an evacuated glass tube. ElectrodeElectrodes are conductors, in the form of wires, rods or plates, through which current enters and leaves the electrolyte. When the current leaves the electrodes it is known as the cathode... More A is connected to the negative terminal of a battery and has a large photosensitive surface and is called the cathodeA cathode is the electrode from which a conventional current leaves the electrolyte. It is the negative part of the cell where reduction takes place. More or emitter. Electrode B is connected to the positive terminal of a battery and is in the form of a wire and is the anodeAn anode is an electrode of a polarized electrical device through which conventional current enters the device. It is the positive part of electrolytes where oxidation takes place. More or the collector.

When the cathode is exposed to light, electrons are emitted from its photosensitive surface. These electrons are attracted to the positive anode and form a current that can be measured with an ammeter or galvanometer as shown in the figure above. The potential difference between the plates AB can be adjusted by connecting and adjusting a rheostat via the battery.

The kinetic energy of the emitted electrons is dependent upon the frequency of light striking the phototube, and the quantity of emitted electrons is dependent on the intensity of the light.

Observations of the Photoelectric Effect:

The amount of current flow (number of electrons) and the kinetic energy of the emitted electrons depend upon:

1. Applied Potential Difference Between the Plates:

For a given photo metal if the frequency and intensity of incident light are kept constant and if the potential difference between the plates is increased then photocurrent also increases until it reaches a maximum (saturated) value.

If the potential applied to the anode is gradually decreased to zero and then made negative, then a certain voltage will be reached where the electrons emitted from the cathode will not have enough energy to reach the anode and will be repelled back to the cathode. At this potential, the current becomes zero and is termed stopping potential. This is the maximum kinetic energy an emitted electron can achieve.

\( \scriptsize K.E \:=\: eV_0\)

2. Intensity of Incident Radiation:

For a given photo metal if the frequency and applied voltage is kept constant with increasing intensity then we observe that photocurrent increases with increases in intensity without change in the stopping potential.

The detected photocurrent plotted versus the applied potential difference shows that for any intensity of incident radiation, whether the intensity is high (I₁) or low (I₂ or I₃), the value of the stopping potential is always the same.

3. Increase in Frequency:

The maximum kinetic energy of ejected electrons increases with an increase in the frequency of light. With a higher frequency of light, the stopping potential becomes more negative (or increases), which implies that the kinetic energy of the photoelectrons also increases.

4. Threshold Frequency and Work Function:

Not all frequencies can cause a photoelectric effect, only those above the threshold frequency (f₀). Also, the maximum kinetic energy of the photoelectrons increases linearly with increasing light frequency. If we extend the graph below the x-axis, the intercept on the kinetic energy axis (y-intercept) represents the negative value of the work function and the minimum energy required for the emission of an electron; this is known as the material’s work function.

Note: Threshold frequency (in Hz) refers to the minimum frequency of light needed to eject electrons from a material, while work function represents the minimum energy (in eV) required to remove an electron from the material’s surface

5. Material of the Photo-Metal:

The experiment is repeated for different photo metal and their threshold frequencies are noted. The variation of frequency versus stopping potential is graphed and it is noticed that threshold frequency varies from metal to metal.

When comparing different materials, the one with a higher work function will also have a higher threshold frequency needed to initiate the photoelectric effect

A metal with a higher work function will yield fewer photoelectrons when exposed to light of the same frequency, because a higher work function means more energy is required to remove an electron from the metal surface, making it less likely for photoelectrons to be emitted.