Physics Lab Experiment 9 PDF

Rajkiya Engineering College, Ambedkar Nagar Physics Lab Department of Applied Science and Humanities NEWTON’S RINGS OBJECT: - To determine the wavelength of sodium light by Newton’s ring. APPARATUS REQUIRED: - A Plano-convex lens of large...

Rajkiya Engineering College, Ambedkar Nagar Physics Lab Department of Applied Science and Humanities NEWTON’S RINGS OBJECT: - To determine the wavelength of sodium light by Newton’s ring. APPARATUS REQUIRED: - A Plano-convex lens of large radius of curvature, opticalarrangement for Newton’s ring Plane glass plate, sodium lamp and travelling microscope. FORMULA USED: - The wavelength  of light is given by the formula: - D2  D2n np  4pR Where, Dn+p = diameter of (n + p )th ring. Dn = diameter of the nth ring. p = an integer number (of the rings) R = Radius of curvature of the curved face of the Plano convex Lens (100 cm). PROCEDURE: 1. If a point source is used only then we require a convex lens otherwise using an extended source, the convex lens L is not required. 2. Before starting the experiment, the glass plate G1 and G2 and the Plano-convex lens should be thoroughly cleaned. 3. The center of the lens L2 is well illuminated by adjusting the inclination of glass plate G1 at 450. 4. Focus the eyepiece on the cross-wire and move the microscope in the vertical plane by means of rack and pinion arrangement till the rings are quite distinct. Clamp the microscope in the vertical side. 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 G2 and Plano- convex lens L2. In this case the lens should be again cleaned. 6. Move the microscope in a horizontal direction to one side of the fringes. Fix up the cross- wire tangential to the ring and note this reading. Again the microscope is moved in the horizontal plane and the cross-wire is fixed tangentially to the successive bright fringes noting the vernier reading till the other side is reached. 7. The radius of curvature of the Plano-convex lens is determined by Boy’s method as discussed below. If an object is placed at the principal focus of convex lens placed over a plane mirror, its image is formed at same point and the distance from the lens is equal to the focal length f of the lens. DESCRIPTION OF APPARATUS: The optical arrangement of Newton’s ring is shown in Figure. Light from a monochromatic source (Sodium lamb) is allowed to fall on a convex lens through a broad slit which renders it  Rajkiya Engineering College, Ambedkar Nagar Physics Lab Department of Applied Science and Humanities into a nearly parallel beam. Now it fall on a glass plate inclined 450 to the vertical thus the parallel beam is reflected from the lower source. Due to the air film formed by a glass plate and a Plano convex lens of large radius of curvature, interference fringes are formed whichare observed directly through a travelling microscope. The rings are concentric circle. OBSERVATIONS: - Value of one division of the main scale = ……………… No. of division on the vernier scale = ……………… Least count of the microscope = ……………… 2 TABLE FOR THE DETERMINATION OF n+p(D n - D 2 ): Ring Micrometer reading Diameter D2 =(a – b)2 (D2n+p– Mean No. D2n) Left end Right end D(a–b (cm2) )(cm) (cm2) MS VS Total MS VS Total (cm2) (a) cm (b) cm 20 18 16 14 12 10 8 6 4 RESULT: - The mean wavelength ( ) of sodium light = ……… Å Standard mean wavelength ( ) = 5893 Å Percentage error = ……… %  Rajkiya Engineering College, Ambedkar Nagar Physics Lab Department of Applied Science and Humanities SOURCES OF ERROR AND PRECAUTION: 1. Glass plates and lens should be cleaned thoroughly. 2. The lens used should be of large radius of curvature. 3. The source of light used should be an extended one. 4. Before measuring the diameter of the rings, the range of microscope should be properly adjusted. 5. Crosswire should be focused on a bright ring tangentially. 6. Radius of curvature should be measured accurately. Questions related to Newton’s ring experiment Q. 1. What do you mean by interference of light? Q. 2. What are interference fringes? Q. 3. Is there any loss of energy in interference phenomenon? Q. 4. What is the physical significance of this phenomenon? Q. 5. What are the essential conditions for observing the interference phenomenonin the laboratory? Q. 6. What are the different classes of interference? Q. 7. What is the construction of sodium lamp? Q. 8. Why does the sodium lamp give out red light in the beginning? Q. 9. Why is the neon gas filled in it at all? Q. 10. How are these rings formed?