Photoelectric current variation with light intensity

Experiment to study the variation of photoelectric current with light intensity using a photocell, galvanometer, and lamp, verifying the inverse square law of l, light.

Apparatus

Photocell, sensitive galvanometer, battery, rheostat, key, electric bulb (preferably a point-type lamp), suitable case for the bulb and photocell, and connecting wires.

Theory

Light intensity is the amount of energy falling per second on a unit area placed perpendicular to the light beam. It is measured in watts per square meter (W/m²).

According to the inverse square law, the intensity of light from a point source decreases with the square of the distance from the source.
Thus,

\propto \frac{1}{d^2}

A photoelectric cell contains a photosensitive cathode and an anode enclosed in a glass tube. When light of suitable frequency strikes the cathode, electrons are emitted. These electrons move toward the anode, producing a photoelectric current.

As the distance between the light source and the photocell increases, the light intensity decreases. Consequently, the photoelectric current also decreases, following the inverse square law.

Procedure

First, draw the circuit diagram as shown in Fig. 15(a).
Next, clean the ends of the connecting wires and arrange the apparatus.
Connect the photocell in series with the galvanometer, rheostat, battery (B), and key (K). Ensure the positive terminal of the battery is connected to the anode of the photocell.
Then, move the lamp as far away from the photocell as possible.
Insert the plug key and adjust the rheostat until a measurable galvanometer deflection is observed.
Record the deflection and measure the distance (d) between the lamp and the photocell.
Repeat the readings by gradually decreasing the distance in equal steps.
For each reading, calculate $1d2\frac{1}{d^2}$ .
Finally, plot a graph between galvanometer deflection (θ) and $1d2\frac{1}{d^2}$ , taking $1d2\frac{1}{d^2}$ along the x-axis. The graph should be a straight line.

Observation and Calculation

NO. OF Obs.	Distance of the lamp from the photocell deflection cm)	proportional to o.fAsGalvanometer	Intensitthe y of light1/d^²(cm^²)
1 2 3 4 5 6

Conclusion

The graph between galvanometer deflection and $1d2\frac{1}{d^2}$ is a straight line.
Hence, deflection is directly proportional to $1d2\frac{1}{d^2}$ , proving that the photoelectric current is proportional to the light intensity.
Therefore, the inverse square law of light is verified.

Precautions

All connections must be tight and clean.
The anode of the photocell should be connected to the positive terminal of the battery, and the cathode to the negative terminal.
A point source of light should be used.
The experiment should begin with the lamp placed farthest from the photocell and then moved closer step by step.

Viva Voce

Q1: What is the photoelectric effect?
A: It is the emission of electrons from a material when light of suitable frequency falls on it.

Q2: What is the threshold frequency?
A: It is the minimum frequency of light that can cause the emission of electrons from a material.

Q3: What is the work function?
A: The minimum energy required to remove an electron from a material is called its work function.

Q4: What is a photoelectric cell?
A: It is a device that converts light energy into electrical energy using the photoelectric effect.

Q5: What is a photon?
A: A photon is a tiny packet or quantum of light energy.

Q6: Write the equation for the energy of a photon.
A:

$E = h f$

where h is Planck’s constant and f is the frequency of light.