Gyan ganga international
Academy, bhopal
2015-2016
To find refractive indices
of water and turpentine
oil
Submitted by :
submitted to:
Ashwin Singh
prasad
Mr.Nagendra
Class: XII ‘D’
Roll no.
CERTIFICATE
This to certify that the project report
entitled “to find refractive indices of
water and turpentine oil” submitted
by Ashwin Singh during the academic
year 2015 – 2016 is a bonafied piece
of work conducted under my
supervision and guidance. The data
sources have been fully
acknowledged. I wish his success in all
his future endeavours.
MR.faizal shareef
(pgt physics)
Mr. d.ashok
(PRINCIPAL)
Acknowledgement
I thank my physics laboratory teacher
guiding me in doing this project. His
constant support and encouragement
helped me a lot in completing this project.
I also thank my chemistry teacher Mr.
Faizal Shareef for helping me with this
project. I also thank the lab assistant and
all other non-teaching staff members for
arranging the apparatus as and when
required.
I also thank my school for providing
such wonderful laboratory facilities.
Index
SR. NO.
1
CONTENTS
OBJECTIVE
PAGE NO.
1
2
APPARATUS
2
3
THEORY
2
4
PROCEDURE
3
5
OBSERVATION
6
6
CALCULATION
7
7
RESULT
8
8
PRECAUTION
8
9
SOURCES OF ERROR
9
10
BIBLIOGRAPHY
10
Objective
TO FIND REFRACTIVE INDICES OF WATER AND
TURPENTINE OIL USING A PALNE MIRROR, AN
EQUICONVEX LENS(MADE OF GLASS OF
KNOWN REFRACTIVE INDEX) AND AN
ADJUSTABLE OBJECT NEEDLE
Apparatus:
A convex lens, an optical needle, a plane mirror, a clamp stand,
a spherometer, a plumb line, metre scale, water and turpentine
oil.
Theory:
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 1
1
= +
F f1 f2
The focal length of the plano-concave lens is,
From Lens Maker’s formula,
1
1
1
=( μ−1 )
−
f1
R1 R 2
(
=(
∴
μ−1 ¿
R1 = R and R2 = ∞ for water lens.
1
R
)
1 1 1
= −
f1 F f2
…(i)
∴
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).
2
The Radius of curvature of the lens, is
l
h
R= +
6h 2
…(iii)
Procedure:
I.
For finding the focal length of convex lens:
(i)
Measure the rough focal length of the convex lens.
(ii) Place the plane mirror with the convex lens placed on
it above the horizontal 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 focal length of the convex lens.
(iii) Try to remove the parallax between the tip of the
object needle and its image tip.
(iv) Note the distance of the tip of the needle from the
centre of the upper surface of the lens. Let it be x 1.
(Use plumb line).
(v) Remove the convex lens and measure the distance of
the tip of the needle from the plane mirror. Let it be
x2. (Use plumb line).
(vi)
Repeat and record all the observations.
II. For finding the focal length of the planoconcave 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.
III. For finding ‘l’:
Determine the pitch and least count of scale of the
spherometer. Place the spherometer on the dried surface
of the convex lens. Turn the screw downwards very
gently till the tip of the screw just touches the lens. Read
and record the reading.
Keep the spherometer’s legs on the base of a paper and
adjusting the central screw, find the pricks A, B and C of
the three legs of the spherometer. Join the centres of the
three pricks and measure the lengths with the half-metre
scale. Note the values of AB, BC and AC.
Fig: (a) to find focal length of convex lens
(b) for the length of legs AB, BC, CA of the
spherometer
Fig: to find focal length of the plano concave lens
Observations
Pitch of the spherometer= 1 cm
Least count of the spherometer = 0.01 cm
Distance between the legs:
(1)
(2)
(3)
AB
BC
CA
= 3 cm
= 3 cm
= 3 cm
Table for calculation of ‘h’
S.N
o
Initial
reading
of the C.S.
on the
convex
lens
No.
of
complet
e
rotation
s
Final
reading
of
the
c.s
on
the
glass
Addition
al
C.S
div.
moved
h=n Mean
x
“h”
pitch
+ m
x
L.C
(a)
(n)
slab
1
62
0
6.5
55.5
2
64
0
4
60
0.55
5
0.6
0.577
5
To measure focal length f‘ of convex lens
Area
between
lens and
plane
mirror
S.N
o
Without
liquid
1
2
Distance of needle tip
from
Tip of the
Upper
upper surface
surface
of the convex
of the
lens
plane
X1
mirror
X2
30.5
31
36.7
37.2
With
water
With oil
1
2
1
2
31.4
37.5
9.4
10.4
31.8
38.1
9.6
10,6
Mean
x=
x1 + x2
2
30.75
36.95
31.6
37.8
9.5
10.5
Focal
lengt
h
f1=
33.8
5
f2=3
4.7
f3=1
0
CALCULATIONS
Mean distance between two legs
l=
AB+ BC +CA
3
=3 cm
Mean h = 0.7557 cm
To find the radius of curvature of the convex lens:
l2 h
R= + =2.27366 cm
6h 2
Measurement of refractive indices of water and
turpentine oil
1)With water between the convex lens and the plane
mirror:
μ1=1+
R
2.27366
=1+
=1.06552
f2
34.7
2) With turpentine oil between the convex lens and the
plane
mirror
μ2=1+
R
2.27366
=1+
=1.22736
f3
10
Result
The refractive index of water is µ1 = 1.06552
The refractive index of turpentine oil is µ2 = 1.22736
Precautions
(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 the plane mirror should be properly
cleaned.
(v) The convex lens of large focal length should be
used.
(vi) The spherometer reading should be taken first on
the surface of the convex lens and then on the
glass slab.
Sources of error:
(i)
The distances x1 and x2 may not be measured
correctly.
(ii) The value of h and l may not be correct.
(iii) Parallax may not be removed properly.
(iv) Backlash error may be acting on the spherometer.
BIBLIOGRAPHY
1. WEBSITES :
 www.wikipedia.org
 www.google.com
 www.yahoo.com
2. BOOKS :
Comprehensive Practical Manual for class XII