pHYSICS

Experiment 5

Aim of the Experiment

To determine the wavelength of source light and radius of curvature of the given convex lens by measuring the diameters of Newton’s rings.

Principle

Rings are fringes of equal thickness. They are observed when light is reflected from a plano-convex lens of a long focal length placed in contact with a plane glass plate. A thin air film is formed between the plate and the lens. The thickness of the air film varies from zero at the point of contact to some value t. If the lens plate system is illuminated with monochromatic light falling on it normally, concentric bright and dark interference rings are observed in reflected light. These circular fringes were discovered by Newton and are called Newton’s rings.

Central dark spot: At the point of contact of the lens with the glass plate the thickness of the air film is very small compared to the wavelength of light therefore the path difference introduced between the interfering waves is zero. Consequently, the interfering waves at the center are opposite in phase and interfere destructively. Thus a dark spot is produced.

Circular fringes with equal thickness: Each maximum or minimum is a locus of constant film thickness. Since the locus of points having the same thickness fall on a circle having its center at the point of contact, the fringes are circular.

Fringes are localized: Though the system is illuminated with a parallel beam of light, the reflected rays are not parallel. They interfere nearer to the top surface of the air film and appear to diverge from there when viewed from the top. The fringes are seen near the upper surface of the film and hence are said to be localized in the film.

The wavelength of monochromatic light can be determined as,

Materials required

A nearly monochromatic source of light (source of sodium light), a plano-convex lens C, an optically plane glass plate P, an optically flat glass plate G inclined at an angle of 45◦ , a travelling microscope with measuring scale and a spherometer.

Procedure

After experimental arrangement, the glass plate is inclined at an angle 45° to the horizontal.This glass plate reflects light from the source vertically downward and falls normally on the convex lens. Newton's rings are seen using a long focus microscope, focussed on the air fil. The cross-wire of the microscope is made tangential to the 8^th ring on the left side of the centre. The readings of the main scale and vernier scale of the microscope are noted. The cross wire is adjusted to be tangential to the 6^th, 4^th, 2^ND, etc on the left and 2nd, 4th, 6th, etc on the right and readings are taken each time. From this the diameter of the ring is found out which is the difference between the readings on the left and right sides. The square of the diameter and hence Dn² and D²n+m are found out. Then wavelength is calculated using the equation.

Observations:

To find Least Count

One main scale division = ............... cm

Number of divisions on Vernier = ...............

L.C = One main scale division/ Number of divisions on vernier=.................................

Observations Table

Calculation

Mean value of D²_m+p - D²_m =.......cm²

Wavelength of light = (D²_m+p - D²_m)/4pR =.............................nm

Result:

Wavelength of light from the given source is found to be = ..........nm

Reference Material

Note: Either Reference 2 or 3 could be used for the experiment.

1. Thermodynamics, Kinetic theory & Statistical thermodynamics, Sears & Salinger, 1988, Narosa

2. Virtual Lab from Iowa State University (https://www.sciencegeek.net/VirtualLabs/SpecificHeatLab.html)

3. Virtual Lab from IIT Bombay (http://vlabs.iitb.ac.in/)

Questions

1. What is interference?

2. What is the condition for constructive interference?

3. What is the condition for destructive interference?