Acknowledgement
I would like to express my special thanks of gratitude to my
physics teacher Mr. Anand Prasad , as well as our vice
principal, Mr. Ravi Joshi and the CBSE board for providing me
with the opportunity to work on this beautiful project.
Secondly, I would also like to thank my parents and friends
who helped me to finish this project within the limited time
frame. Finally, I would like to thank everyone without whose
help I could not have completed my project successfully.
INTRODUCTION
Refractive index, measure of the bending of a ray
of light when passing from one medium into another. If i is
the angle of incidence of a ray in vacuum (angle between the
incoming ray and the perpendicular to the surface of a
medium, called the normal) and r is the angle
of refraction (angle between the ray in the medium and the
normal), the refractive index n is defined as the ratio of the
sine of the angle of incidence to the sine of the angle of
refraction; i.e., n = sin i / sin r. Refractive index is also equal
to the velocity of light c of a given wavelength in empty space
divided by its velocity v in a substance, or n = c/v.
Experiment
Aim: To find the refractive indices of a) water and b) oil
transparent using a plain mirror, an equiconvex lens (made
from a glass of non-refractive index) and an adjustable object
needle.
Apparatus: A convex lens, a plain mirror, water, oil, clamp
stand, an optical needle, plumb line, knitting needle, half
meter scale, glass slab and spherometer.
Theory:
Let us consider f1 and f2 to be the focal length of the glass
convex lens and liquid lens respectively and let F be the focal
length of their combination, then
1/F=1/f1+1/f2
From the lens maker’s formula
We have,
1/f2=(n−1)[1/R1−1/R2]
Simplifying further, we get
1/f2=(n−1)/R
Inserting values of f2, n can be calculated.
Procedure
For focal length of convex lens
1. Find rough focal length of convex lens.
2. Place a plain mirror on the horizontal base of the iron
stand and then a convex lens on the plane mirror.
3. Hold the needle in the clamp stand and adjust its position
on the stand such that there is no parallax between the
tip of the needle and its image. Ensure that the tip of the
needle appears touching the tip of the image.
4. Measured distance between tip and upper surface of the
lens by using a plumb line and half metre scale. Also
measure the distance between tip of needle and upper
surface of the mirror. Take the mean of these two
readings. This mean distance will be equal to the focal
length of the convex lens (f1).
For focal length of the combination
5. Put a few drops of water on the plain mirror and put the
convex lens over it with its same face above as before.
The water spreads in a form of layer and acts as a plano-
concave lens as shown in the figure.
6. Repeat the steps 3 and 4 to determine the equivalent
focal length of combination.
7. Record the observation as given below in table 1.
8. Repeat steps 5, 6,7 for other transparent liquid (oil).
For radius of curvature of convex lens surface
9. Determine the pitch and the least count of the
spherometer.
10. Remove the convex lens and dry it completely. Put the
spherometer on this lens surface.
11. All the three legs of spherometer should place
symmetrically on the lens and adjust the central screw tip
just to touch the surface of the lens.
12. Remove the spherometer from surface of lens and place
on the plain mirror and turn the central screw so that its
tip touches the plain mirror surface and record the
reading as shown in table 2.
13. Repeat the steps 10 and 11 three times.
14. Obtain the impressions of three legs of spherometer on
paper and mark them and their average distances.
l= l1+l2+l3
3
Observations
1. The rough focal length of convex lens = _____ cm
2. Table 1, to find the focal length of the lens and
combination.
Arrangement Distance of needle tip Focal
From lens From Mean length
surface x1 plane X=(x1+x2)/2 x (cm)
(cm) mirror x2
(cm)
Without
liquid
With water
With other
transparent
liquid
3. Table 2, to find h.
S. no Initial C.S.R No. of Final C.S.R m=(a-b) if h=n x
on convex complete on plane a>b pitch +
lens (a) rotations mirror =(100+a-b) m x L.C
(n) surface if a<b
1.
2.
3.
Mean value of h = ________mm =________cm
Calculation
1. l= l1+l2+l3 = _______ cm
3
2. R=l2 + h2 = _______ cm
6h 2
3. f2 = ff1 = _______ cm
f1 – f2
4. n = 1+ R = ________ cm
f1
Results
1. The refractive index of water = _______
2. The refractive index of other liquid = _______
Precautions
1. Only a few drops of liquid should be taken so as not to
thicken the layer.
2. The parallax should be removed tip to tip.
3. The liquid taken should be transparent.
4. Only legs of spherometer should be vertical.
5. The plane mirror should be clean and fully shining
surface.
Sources of Error
1. The liquid may not be quite transparent.
2. The parallax may not be fully removed.
3. The spherometer legs may not be placed symmetrical on
the surface of convex lens.
4. The tip of the central screw may not just touch the
surface of lens or mirror.
