Page 88 - Physics - XII

17. 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’.
18. Now, place the spherometer on a glass slab and rotate the screw in downward direction again till 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’.
19. 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

20. Repeat the steps 15 to 19 and take two more observations.
21. Record all the observations in the observation table.
22. 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.
AB BC CA+ +
Hence, l =
3
Observations
Table for focal length of the convex lens (f )
1
S. Distance of tip of the needle from Focal length of convex lens
No. Upper surface of Upper surface of ab

the convex lens, the plane mirror, f 2
1
a (cm) b (cm) (cm)

1.

2.

3.

Mean focal length f = _____ cm
1
Table for focal length of combination of convex lens and plano-concave lens formed by
transparent liquid (F).

S. Distance of tip of the needle from Focal length of combination
No. Upper surface of the Upper surface of the plane F a b (cm)

convex lens a′ (cm) mirror, b′ (cm) 2

1.

2.

3.

Mean focal length F= _____ cm

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