TIME TABLE FOR B1:-

 TIME TABLE FOR B2:-

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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.

PHYSICS EXPERIMENT 10

EXPERIMENT NO. 10

Object: To measure the numerical aperture of the given an optical fiber.

Apparatus Required: Diode Laser Source, Fiber Holders, Optical Fiber, and Base With Rotational Mount, Holders And Bases.

Theory: Numerical aperture is a basic descriptive characteristic of a specific fiber. It represents the size or degree of openness of the input acceptance cone. Mathematically it is defined as the sine half angle of the acceptance cone.

Using snell’s law, the maxima angle with in which light will be accepted into and guided through fiber is

NA=Sin(ө_a)= (n₁²- n₂²)^1/2

Where              ө_a is the numerical aperture and n₁ and n₂ are the refractive indices of the core and the cladding. If the incident angle ө < ө_a, the ray undergoes multiple internal reflections at core and cladding interface and it is called the guided ray. If ө_a < ө, the ray undergoes only partial reflection at core cladding interface.

In short length of straight fiber, ideally a ray launched at angle ө at the input end should come out at the same angle ө from output end. Therefore, the far field at the output end will also appear as a cone of semi angle ө_a emanating from the fiber end.

DIAGRAM:- 

Procedure:

1.    Mount Laser source, objective and detector on the respective holders.

2.    Mount both the ends of the optical fiber on the fiber holders.

3.    Align the difference objects as per the setup shown below.

4.    Couple the light from the laser source onto one of the fiber ends using a microscopic objective (provided with the kit).

5.    Place the screen (sheet having circular markings) at some distance from the output end of the fiber such that it is perpendicular to the axis of the fiber. Now move the screen towards or away from the output end of the fiber such that circular beam emanating from the fiber end covers the (1^st or 2^nd or 3^rd) circle on the screen.

6.    Measure the distance between the output end of optical fiber and screen. Let this be L, also measure the diameter of the circular spot formed on the screen. (Diameter is mentioned in mm). Let it be D.

7.    Use the formula

Observations table:

S.No.	Diameter of Laser Spot D mm	Distance between Optical Fiber and Screen L mm			NA=Sin ө

Result:

Numerical Aperture of given optical fiber is=……………………………………

Percentage Error:-

                                           (Standard value~ Observed Value) X 100

                  % ERROR =

                                                                    Standard Value

Precautions:

1.    Reading should be taken carefully.

2.    Direct viewing of laser light should be avoided.

3.    Laser light should be incident normally on the Screen.

Viva voce

   1. Define optical fiber.

        Ans. It is a cable which carries optical signal.

2. On which phenomenon light propagation take place in optical fiber

        Ans. Total internal reflection.

3.  What is total internal reflection?

      Ans. When light travels from denser medium to rarer medium and   angle of incidence is greater than critical angle then light reflects back into same medium this phenomenon is called Total internal reflection.

4.  Define  angle of acceptance

      Ans. Angle at which light enter into optical fiber and Total Internal Reflection   takes place.

5.  What is physical significance of numerical aperture.

    Ans. Light gathering ability of the fiber.

6.  What are the types of optical fiber

    Ans. There are two types of fiber (1) step index fiber (2) graded index fiber.

7.  What is fractional refractive index?

     Ans. It is the ratio of difference of the refractive indices of core and cladding to the refractive index of core i.e.,

8.  Which light source is used in experiment

    Ans. Diode Laser

9.  LASER stands for what?

    Ans. Light Amplification by Stimulated Emission of Radiation

10.     How many type of fiber losses are there?

        There are three types of fiber losses

(1)    Absorption

(2)   Rayleigh Scattering

(3)   Geometrical losses