This document describes a physics experiment conducted by Abhishek Dhinge to determine the refractive indices of water and turpentine oil using a plane mirror, convex lens, and adjustable object needle. The experiment involves measuring the focal lengths of the convex lens alone and in combination with plano-concave lenses of water and turpentine oil formed between the lens and mirror. The radius of curvature of the convex lens is also measured using a spherometer. Calculations using lens maker's formula and measured values yield the refractive indices of water as 1.06552 and turpentine oil as 1.22736.

2. ATMA MALIK INTERNATIONAL SCHOOL
KOKAMTHAN,kopargaon
PHYSICS PROJECT
TOPIC
C C C
:NDIAN MAT
PERPARED BY: Abhishek Dhinge
GUIDED BY:Mr.Praveen Dhumal

3. I wish to express my deep gratitude and sincere thanks to the
Principal Mr. Namdeo Dange, for his encouragement and for all the
facilities that she provides for this project work. I sincerel y
appreciate this magnanimity by taking me into his fold for which I
shall remain indebted to her . I extend my heartily thanks to Mr.
Prveen Dhumal , Physics Teacher, who guided me to the successful
completion of this project.

4. 1. AIM
2. APPARATUS
3. THEORY
4. PROCEDURE
5. DIAGRAMS
6. OBSERVATION
7. CALCULATIONS
8. RESULT
9. PRECAUTIONS
10.SOURCE OF ERROR

5. “ To find refractive indices of water and turpentine
oil using a plane mirror, a equiconvex lens

6. (made from a glass of known refractive index)
and an adjustable object needle’’

7.  A convex lens
 An optical needle
 A plane mirror
 A clamp stand
A spherometer
A plumb line
Meter Scale
Water
Turpentine oil

8. Let’s add small amount of water on a flat , plane surface and
place a convex lens over it. This forms a plano-concave lens of
water between the lower surface of convex lens and plane
mirror.
Let f1 and f2 are the focal lengths of water lens and convex lens
respectively , then focal length of the combination is :
1
𝑓
=
1
𝑓1
+
1
𝑓2
The focal length of the plano-concave lens is,
1
𝑓
=
1
𝑓1
−
1
𝑓2
i)(

9. From lens Maker’s formula ,
1
𝑓1
= 𝜇 − 1 (
1
𝑅1
−
1
𝑅2
)
= 𝜇 − 1
1
𝑅
∴ R1 = R and R2 = ∞ for water lens .
∴ The refractive index of water is , 𝜇 = 1 +
𝑅
𝑓1
….(ii)
( where ‘R’ is the radius of curvature of the concave
surfaces of the plano– concave lens .)
The radius of curvature of the lens is , 𝑅 =
𝑙2
6ℎ
+
ℎ
2
…..(iii)

10. 1. FOR FINDING THE FOCAL LENGTH OF CONVEX LENS:-
I) Measure the rough focal length of convex lens
II) place the plane mirror with the convex lens placed on it above the horizont
al base of a clamp stand horizontally as its tip lies vertically above the optical
centre of the lens. Adjust the needle at a height a little more than the rough
f ocal length of the concave lens.
III) Try to remove the parallax between the tip of the object needle and its
image tip.
IV) Note the distance of tip of the needle from the centre of the upper surface
of the lens. Let it be X1 (use plumb line )
V) Remove the convex lens and measure the distance of the tip of the needle
fro m the plane mirror. Let it be X2 (use plumb line) VI) Repeat and record all
the observations.

11. VI) FOR FINDING THE FOCAL LENGTH OF PLANO-CONVEX LENS :-
Pour few drops of water over the plane mirror and place the convex lens over it.
Repeat steps ii) to iv) as done above.
Repeat the procedure with turpentine oil also.
2. FOR RADIUS OF CURVATURE OF CONVEX LENS SURFACE ‘L’ :-
Determine the pitch and least count of scale of the spherometer. Place the sphero meter
on the dried surface of convex lens . Turn the screw downwards very gent ly till the tip of
the screw just touches the lens . Read the record the reading.
Keep the spherometer’s legs on the base of a paper and adjusting the central scre w ,
find the pricks A,B and C of the three legs of the spherometer. Join the center s of the
three pricks and measure the length with the half – meter scale. Note the values of AB ,
BC, and AC .

14. Pitch of the spherometre = 1cm
Least count of the spherometre = 0.01cm
Distance between the legs :-
1) AB = 3cm
2) BC = 3cm
3) CA = 3cm Table for calculation of ‘h’
S.N
O.
Initial reading
of the C.S. on
the convex lens
a( )
No. of
complete
rotations
Final
readings of
the C.S on
the glass
slab
Addition
al C.S
div.
Moved
h = n ×
pitch + m
× L.C
Mean “h”
1 62 0 6.5 55.5 0.555
0.5775
2 64 0 4 60 0.6

15. Area
between
lens and
plane
mirror
S.no. Tip of the
upper surface
of the concave
lens X
1
Upper
surface of
the plane
mirror X
2
Mean
X = X
1+
X
2
/2
Focal
length
Without
liquid
1 30.5 31 30.75
f1 = 33.85
2 367 37.2 36.95
With water
1 31.4 31.8 31.6
f2 = 34.7
2 375 38.1 37.8
With oil
1 9.4 9.6 9.5
f3 = 10
2 10.4 10.6 10.5
Distance of needle tip from
To measure focal length ‘f’ of convex lens

16. 𝐴𝐵+ 𝐵𝐶+ 𝐶𝐴
3
= 3 𝑐𝑚
𝑅 =
𝑙2
6ℎ
+
ℎ
2
= 2.27366 𝑐𝑚

17. 𝜇 = 1 +
𝑅
𝑓2
= 1 +
2.27366
34.7
= 1.06552
𝜇 = 1 +
𝑅
𝑓3
= 1 +
2.27366
10
= 1.22736

18. 18
 The refractive index of water 𝜇1
= 1.06552
 The refractive index of turpentine
oil is 𝜇2 = 1.22736 .

19. i. The screw of the spherometer should be moved in one direction only
to avoid backlash error.
ii. The screw should be moved just to touch the surface of the convex
lens or the glass slab.
iii.The parallax should be removed properly.
iv.The lens and plane mirror should be properly cleaned. v. The convex
lens of large focal length should be used.vi.The
spherometrereadingshould be taken first on the s urface of the convex
lens and then on the glass slab.

20. The value of ‘h’ and ‘I’ may not be correct.