PHYSICS EXPERIMENT 11

Experiment No.-11

Object:- To verify Inverse square law of light using a photo cell.

Apparatus Required:- Optical bench, Photocell housed with Red and Black sockets, lamp house with lamps, DC Microammeter.

Theory:- The Photoelectric emission may be regarded as a phenomena of liberation of an electron at the surface of a metal when a photon of light having energy above threshold energy (metal work function) incident on a metallic surface and transfer the enough energy to the electron to escape through the potential barrier layer. The photo cell can be considered as the generation of a voltage across a circuit element under illumination.

Let P be the illuminating Power of a source so the intensity of illumination I due to it at a distance r would be 

I= P/r²

Since the photo electric current (θp) produced is directly proportional to the intensity of illumination ie.

I α θp

I=Kθp

Where K is constant, hence I= P/r² = Kθp. Since P and K are constant hence the relationship between 1/r² and θp is straight line which verifies the inverse square law of radiation.

DIAGRAM:- 

Procedure:-

1.    Arrange the optical bench in such a way that both the lamp and the photo cell are at the same level as shown in the figure.

2.    Make the connection of photo cell to microammeter Red to (+)ve and black to (–)ve terminal of the microammeter.

3.     Adjust the distance of the lamp such that we will get the microammeter reading

4.    Then decreases the distance in step of 5cm and each time note the reading in microammeter and note your observation in table-1.

5.     Draw the curve between 1/r² and d. 1/r² on x axis and θp at y axis.

Observation table:-  

S.N.	Distance of lamp from cell in cm ‘r’	1/r2	Reading in microammeter (θp) uA
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.

   

Result:-  

The graph between θp and 1/r² is a straight line. It show that microammeter reading is inversely proportional to the square of the distance from the source. but deflection is directly proportional to the intensity of illumination of the surface. Hence we can say that intensity of illumination varies inversely square of the distance from the source. Thus inverse square law is verified.

Precautions:

1. Light should fall on normally on the photocell

2. The photocell should not be exposed to light for a long time continuously.

3. A Cover should be placed on the photocell to protect it.

Viva Voce:

Q.1 What is photoelectric effect?

Ans: When light is falls on metal surface, an electron is emitted from a metal if the energy of the photon is greater than the work function of the metal.

Q.2 What is the photo cell?

Ans: A photocell is a type of resistor. When light strikes the cell, it allows current to flow more freely. When dark, its resistance increases dramatically.

Q.3: Does the photo electric current depend on frequency of light and intensity of light? How?

Ans: The photons of a light beam have a characteristic energy proportional to the frequency of the light. In the photoemission process, if an electron within some material absorbs the energy of one photon and acquires more energy than the work function (the electron binding energy) of the material, it is ejected. If the photon energy (Frequency) is too low, the electron is unable to escape the material. Increasing the intensity of the light beam increases the number of photons in the light beam, and thus increases the number of electrons excited, but does not increase the energy that each electron possesses. The energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the energy or frequency of the individual photons. It is an interaction between the incident photon and the outermost electron.

Q.4: Define the illuminating power, Intensity of illumination?

Ans: Illumination power is defined as the intensity per unit area. Intensity is defined as no. of photon incident per unit area. 

Q.5:Explain the construction of photo cell?

Ans: This is Selenium Photocell. This consist of metal base plate mainly Aluminum, steel or brass. On This base plate a very thin layer of special grade selenium is deposited followed by a deposition of light transparent protective film of cadmium or cadmium Oxide. The light sensitive face of a selenium photocell is either brown or bluish brown in appearance and has a narrow silvery collector strip. This strip serves as a negative terminal. The back of base plate is also coated with the same material as the narrow strip and serves as a positive terminal.