Apparatus:
Newton’s ring apparatus, convex lens, Plano-convex lens, screen with hole, a source of sodium light, traveling microscope, spherometer.
bservations and Calculations:
- Determination of radius of curvature of lens:
Pitch of the spherometer screw =
Number of divisions on the circular scale = N =
Least count of the spherometer = pitch / N
Distance between the two legs (i) (ii) (iii)
| No of Obs | Reading of | spherometer on | Meanh | Radius of curvature | ||
| Convex surface | Place surface | Difference h | ||||
| 123 | ||||||
- Microscope Readings:
Least count of microscope = smallest scale division =___________cm
No. of divisions on the circular scale
| No. ofObs | Ring No. | Reading of RingM | Reading of RingN | | ||||||
| n | m | Left end | Right end | DiameterR Dm | Left end | Right end | DiameterR Da | |||
| 123 | ||||||||||
Result:
The wavelength of sodium light = oA
Procedure:
Take a Plano-convex lens L and optically glass plate G and a convex lens of small focal length Li, and then clean their surfaces with spirit. Arrange the apparatus as shown in fig, Fix the convex lens Li of small focal length in the hole and place the source of sodium light at focus to get parallel rays. Place the glass plate G inclined at an angle of 45̊ and adjust its position so that light from Li falls on it fully. Place the plate P below G on a blank paper so that no light can reflect from its lower surface. Place the piano -convex lens L on the plate P so that the point of contact 0 of the two surfaces remains vertically below the microscope. Mount the microscope above G and focus its eye-piece on the cross-wire. Adjust the position of the microscope so that it lies vertically above the center of the lens L focus it so that circular alternate dark and bright rings are clearly visible.
Set the point of intersection of cross-wires at the center of the ring system. The first few rings will not be distinct. Now move the horizontally and set the cross-wire tangentially to the clear 20th dark ring. Note the reading on the scale. Bring the cross-wire on the other side of the diameter of the same 20th dark ring and again note the reading. The difference of these two readings gives the diameter of the 20th ring. Similarly measure the diameters of 16th, 12th, 8th and 4th dark rings and note these readings as shown in the observations. Calculate the wave-length of light used taking various combinations of ring number by the given formula, where D is the diameter of rings n and m are any number of rings.
Measure the radius of curvature of the piano convex lens L with spherometer. Let it be R, Put this value of R in the above relation.
Precautions:
- The number of rings should be within the range of microscope.
- The lens used should be of large focal length.
- The source should be at the principal focus of lens Li so that a parallel beam of light is obtained.
- The point of intersection of the cross-wires should exactly coincide with the center of the ring system.
- Light should be incident normally on the lens L.
- The microscope should be focused on the point of contact.
- The radius of curvature of the face of the lens in contact with the glass plate should be measured.
VIVA VOCE
Q.1.What are Newton rings?
Ans. The thin film of air between the tens and the glass surface produces circular interference fringes or rings, due to repeated reflections from the glass plate and the lens surface.
Q.2. What makes a piece of glass visible in water?
Ans. Difference between their refractive indices.
Q.3. What is interference of Light?
Ans. When two narrow real or virtual light sources emit coherent wave trains with constant phase difference, then the trains reinforce each other at some points and cancel each other points, this results in the production of dark and bright fringes and the phenomenon is known as interference.
Q.4. What type of fringes are obtained when white light used?
Ans. Colour circular fringes are obtained.
Q.5. Why is the sodium light used in this experiment? Ans. It is nearly monochromatic light.