Page 127 - Physics - XII

Determinati on of focal length of the plano-concave lens formed by water or oil

9. Fill the open space between plane mirror and lens with water or oil using dropper. Now, place the
convex lens over.
10. Now, repeat steps 4 to 6 to fi nd the focal length of the combination of plano-concave lens (f) and
convex lens (f ). Let a' and b' be the distances of the upper surface of the convex lens and the plane
1
mirror from the tip of needle [Fig. 3.2], then the focal length of the combination of convex lens and
plano-concave lens is given by

a b
F
2
11. Repeat the steps 9 and 10 at least two times more and take observations.

12. Note all the observations in the observation table.
13. After determination of focal length of convex lens f and focal length of combination of convex lens
1
and plano-concave lens formed by water or oil F, the focal length of plano-concave lens f can be
calculated as
1 1 1
f F f 1

Determinati on of radius of curvature (R) of the surface of convex lens

14. Calculate the pitch and least count of the spherometer.
15. Note the reading of the circular scale, i.e. the number of division of circular scale which is coinciding
with vertical main scale. Let this reading be ‘a’.

16. Now, place the spherometer on a glass slab and rotate the screw in downward direction again til it just
touches the surface of the glass slab. In this position, the tip of the screw just touches its image on the
glass slab. Again note the circular scale reading in this position. Let it be ‘b’.
17. Taking initial circular scale reading ‘a’ as the reference mark; count the number of complete rotations
made by circular scale between the initial and fi nal readings a and b of the circular scale. If ‘n’ be the

number of complete rotations made by circular scale, then the value of h is given by
h = n × pitch+ (a – b) × least count (if a > b)
and h = n × pitch + [(100 + a) – b] × least count (if b > a)
18. Repeat the steps 15 to 17 and take two more observations.
19. Record all the observations in the observation table.

20. Now, press the spherometer gently on the practical notebook to get the impressions of tips of the
three legs of the spherometer A, B, and C. Join A, B, and C to form the triangle and measure AB, BC,
and CA. Take the mean of AB, BC, and CA which gives the mean distance ‘l’ between the two legs of
the spherometer.

Hence, l AB BC CA
3

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