Newtons Ring

To Find The Wavelength of Sodium Light using Newton’s Ring

1. PHYSICS EXPERIMENT
By:
Rashi Pandey(19PCS5024)
Rahul Kumar Upadhyay(19PCS5022)
Aayush Joshi (19PCS5001)

2. AIM
To Find The Wavelength
of Sodium Light using
Newton’s Ring

3. APPARATUS REQUIRED
1.A plano-convex lens of large radius of
curvature
2.optical arrangement for Newton’s rings
3.glass plate
4.sodium lamp
5.travelling microscope.
6.Spherometer

4. DESCRIPTION OF APPARATUS
The optical arrangement for Newton’s ring is shown in
figure. Light from a monochromatic source (sodium
lamp) is allowed to fall on the convex lens through a
broad slit which renders it into a nearly parallel beam.
Now it falls on a glass plate inclined at an angle 45° to
the vertical, thus the parallel beam is reflected from the
lower surface. Due to the air film formed by a glass plate
and a plano convex lens of large radius of curvature,
interference fringes are formed which are observed
directly through a travelling microscope. The rings are
concentric circles.

6. Travelling microscope
A travelling microscope is an instrument for
measuring length with a resolution typically in
the order of 0.01mm .

The least count of Travelling Microscope is :- 0.001
cm
You can take readings by this formula :-
:- Main scale reading + (Best Coinciding division of
vernier scale with main scale division)×Least count.

7. DIFFERENT PARTS
OF TRAVELLNG
MICROSCOPE

8. SPHEROMETER
Spherometer :- It is a precision
instrument to measure very
small lengths which were
primarily used by opticians to
measure the curvature of the
surface of a lenses

10. NEWTON'S
RINGS

11. SODIUMLAMP

12. Consider a ring of radius ‘r‘ due to thickness ‘t‘ of air film as shown in
the figure given below:








R is the radius of the circle, O is the center of the circle, AC is the
diameter, DE is the chord, r is the
distance between D and E, t is the height between the chord of the
circle and the plane glass plate (AB).
According to geometrical theorem, the product of intercepts of
intersecting chord is equal to the product
of sections of diameter then
THEORY AND FORMULA USED

17. EXPERIMENTAL SETUP

18. SETTING UP OF
CROSSWIRE ON NEWTON
RING

19. PROCEDURE
i) Set up the apparatus.

ii) Before starting the experiment, the glass plates and
the plano convex lens should be thoroughly cleaned.

iii) The centre of lens gets well illuminated by adjusting
the inclination of glass plate at 45°.

iv) Focus the eyepiece on the cross-wire and move the
microscope till the rings are quite distinct. Clamp the
microscope in the
vertical side.

20. v) According to the theory, the centre of the interference
fringes should be dark but sometime the centre appears
white. This is due to the presence of dust particles
between glass plate and plano-convex lens. In this case
the lens should be again cleaned.

vi) Move the microscope in a horizontal direction to one
side of the fringes. Fix up the crosswire tangential to the
ring and note the 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 readings till the other side is reached.

vii) The radius of curvature of plano-convex lens is
determined using spherometer.

21. Virtual laboratory

22. Virtual laboratory

23. Virtual laboratory

24. Least count
Least count (LC) = Smallest reading on main scale/Number of
divisions on the vernier scale

LC = 0.5/50

LC = 0.01 mm

LC = 0.001 cm

25. OBSERVATIONS
Finding Value of r:
To calculate value using
spherometer:



Where
r= radiusof curvature of lens
l= distance between two legs of spherometer
h= calculated value
We have taken the value r=100cm in this experiment .
The experiment has been performed in amrita virtual lab,
which provided the option to set value of r between 1 to
100 cm.

26. OBSERVATIONS

27. CALCULATION
S
here, p = 6
Calculating D²(n+p)-D²n:


D²seven-D²one = 0.053291
D²eight-D²two =
D²nine-D²three =
D²ten-D²four =
D²eleven-D²five =
D²tweleve-D²six =
D²thirteen-D²seven =
Mean= 0.05808871429
Using, λ = (D²n+m-D²n)/ 4Rp
λ = 0.05808871429/ 4(100)(6)
λ = 0.000005420363095 cm
=542.0363095 Angstrom
0.053291
0.055096
0.05632
0.055545
0.066297
0.055272
0.0648

28. GRAPH

29. RESUL
T
Wavelength of sodium light calculated by
Newton’s rings experiment = 542.036A
0

:-
= {(589-542.036)/589}*100%
=7.97%431

30. PRECAUTIONS AND SOURCES
OF ERROR
•The lens should be of large radius of curvature.
•The sources of light used should be an extended
one.
•Before measuring the diameter of the rings , the
range of the microscope should be properly
adjusted.
•Glass plates and lens should be cleaned thoroughly.
•Crosswire should be focused on a bright ring
tangentially.
•Radius of curvature should be measured accurately.

31. APPLICATIONS OF NEWTON’S
RINGS
•To measure the refractive index of a
substance placed beneath the same lens.
•To check the thickness of a surface and to
check whether a surface is uniform.
•The interference technique of Newton’s rings is
widely used for the quality control of optical
surfaces because the precision obtained with this
method proves to be very satisfactory. The
dimensions of the permits calculation of the radii of
curvature of the analyzed surfaces and deformation
of the interference pattern can be utilized to
calculate other parameters such as astigmatism.

32. BIBLIOGRAPHY
•www.google.com
•www.wikipedia.org

33. THANK YOU