Page 74 - Physics - XII

According to sign convention, u and v are positive for concave lens. Since v > u, so f will always
be negative.

Procedure

1. Estimate the rough focal length of the convex lens L by focussing a distant object on wall or
1
white paper.
2. Mount the convex lens L in a clamp and place it on one central upright of the optical bench, also mount
1
two needles on the two other uprights on either side of the convex lens L in such a way so that keeping
1
tips of the needles at the same height as the centre O of the convex lens. Mark one needle as object
1
needle O and the other as image needle I.

3. Place the object needle O between f and 2f of the convex lens. Adjust the image needle I beyond 2f
on the other side of the lens such that the tip to tip parallax is removed between image needle I and
the image of the object needle O as shown in Fig. 4.1(a). Note the positions of O, L , and I on the
1
bench scale.
4. Place the given concave lens L between I and L without disturbing O and L . Shift the image needle
1
2
1
back and adjust its position so that the parallax is again removed tip to tip between image needle I'
and the new image of the object O formed by the combination of the two lenses L and L . Note this
1
2
position of Ι' of the image needle and L on the bench scale.
2
5. Repeat the experiment for four more sets of observations by changing the position of L with respect
2
to L or by changing the position of O with respect to L or both.
1 1
6. Find the index correction between concave lens L and the image needle I.
2
Observations
1. Rough focal length of the convex lens = _____ cm.
2. Actual length of the index needle = x = _____ cm.
3. Observed distance between the concave lens and image needle when index needle is placed between

them = y = _____ cm.
4. Index correction for u as well as v = x – y = _____ cm.

Table for determination of focal length of concave lens

Position of Observed Corrected distance uv
S. f

No. O L at O 1 I' L at O 2 I' u = O I v = O I' u v uv
2
2
1
2
(cm) (cm) (cm) (cm) (cm) (cm) (cm) (cm) (cm) (cm)
1
2.
3.
4.

5.

Mean f = _____ cm

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