PHYSICS EXPERIMENT 8

EXPERIMENT NO. 8

Object: - To determine the wavelength of sodium light by Newton’s Ring.

Apparatus: - A Plano-convex lens of large radius of curvature, optically arrangement for Newton’s rings, plane glass plate, sodium lamp and traveling microscope.

Theory:

The optical arrangement for Newton’s Ring is shown in Fig.(1). A wedge shape air film in formal between Plano-convex lens and glass plate. Interference take place between light reflected from concave surface of lens and upper surface the plate as shown in Fig.(2). In this Experiment the Fringes are of equal thickness i.e. why fringes are circular as shown in Fig.(3).

In this experiment path difference between reflected rays from lens and plate is . Locus of points having the same thickness then full on a circle having. Its center at the point of contact.  Thus, the thickness of the film is the same at all points on any circle having 0 as the center of the fringes one therefore circular as shown in Fig.(3).

DIAGRAM:- 

Formula used: - The wavelength of Sodium light is given by,

                         

          

Where                     D_n+p = Diameter of (n+p) ^th ring.

                               D_n   = Diameter of n^th ring.

                               p     = An integer numbers of the ring.

                 R     = Radius of curvature of the curved face of the Plano convex lens.

Procedure: -

1.        If a point source is used only then we require a convex lens otherwise using an extended source, the convex lens is not required.

2.        Before starting the experiment, the glass platesG1, G2 & the Plano convex should be       thoroughly cleaned.

3.        The center of lens L2 is well illuminated by adjusting the inclination of glass plate G1 at 45 deg.

4.        Focus the eyepiece on the crosswire and move the microscope in the vertical plane by means of rack & pinion arrangement till the rings are quite distinct clamp the microscope in the vertical scale.

5.        According to the theory, the center of the interference fringes should be dark but sometimes the center appears white, this is due to the presence of dust particles between glass plate G2 and Plano convex lens L2.in this case lens should be again cleaned.

6.        More the microscope in a horizontal direction to one side of the fringes.

  Fix up the cross wire tangent to the ring and note this reading. Again the microscope is moved in the horizontal plane and the crosswire is fixed tangentially to the successive bright fringes noting the vernier reading till the other side is reached.

Observations: - 

Least count of the travelling microscope = 0.01mm

  Table for determination of [D²_(n+p) ^–    D²_n ]

No of rings	Microscope reading		Diameter       (a-b) mm	D2=(a-b) 2	D2 n+p–    D2n	Mean mm2	P
	Left end   (a) mm.	Right end   (b) mm.
16
14
12
10
8
6
4
2

Calculations:-

                      The wavelength of sodium light is given by-

Result: - The mean wavelength of sodium light = --------------------Å

Percentage error

                                              (Standard value~ Observed Value) X 100

                  % ERROR =

                                                                    Standard Value

Precautions: -      

1.    The lens used should be large radius of curvature.

2.    Before measuring the diameter of rings the range of the microscope should be properly adjusted.

3.    Cross wire should be focused on a bright ring tangentially.

Viva voce

Q.(1). Define Interference.

Ans. Redistribution of energy is called Interference.

Q.(2). What are the types of interference?

Ans. There are two Types of interference

(1)Division of wave front and            (2) division of amplitude

Q.(3). On which type of interference does Newton ring based.

Ans. Division of amplitude.

Q.(4.) Which type of light is used in experiment?

Ans. monochromatic source (Sodium light)

Q.(5). Why fringes are circular?

Ans In this experiment fringes are of equal thickness due to the locus of the fringes of equal thickness lies on a circle hence it is circular.

Q.(6). What is the standard wavelength of light used in experiment?

Ans. 5896 A⁰

Q.(7).What is the condition for dark central spot?

Ans. At the central spot thickness of film is zero.

Q.(8). What are the uses of this experiment?

Ans. (1) To determine the refractive index of liquid,

        (2)To determine the wavelength of light

        (3)To determine the radius of curvature of lens.

Q.(9).Which type of lens is used in experiment.

Ans. Plano convex lens

Q.(10). What happened when experiment would be performed by white light.

Ans. A few distinct colored rings be seen