Page 67 - Physics - XII

The object needle is placed at a distance equal to 1.5 times the rough focal length of the convex lens. Adjust
the position of convex mirror M so that a real and an inverted image of object needle O is formed at O
itself. In this position there is no parallax between object needle O and its image at O. In such a situation,

the light rays are incident normally over the convex mirror to retrace their path. In the absence of convex
mirror, these rays would have met at centre of curvature C of the convex mirror.
The distance PC gives the radius of curvature R of the convex mirror.
To locate the position of C, the convex mirror M is removed without disturbing the positions of lens L and
object needle O. An image needle is placed at the position of the image I' of the object O, formed by the
lens L by using parallax method, i.e. remove the parallax between tip of the inverted image of O needle and
tip of the image needle at I' as shown in Fig. 2.1(b). Now, measure PC, then

PC = PI' = R

R PC P I
and f =
2 2 2

Procedure
1. Find the rough focal length of the given convex lens by following step 1 of the previous experiment.

2. Place three uprights mounted with object needle O, convex lens L, and convex mirror M on the optical
bench. Adjust the heights of the uprights so that the tip of the object needle, optical centre of the convex
lens, and pole of the convex mirror lie on the same horizontal line parallel to the optical bench.
3. Now, adjust the position of the object needle in such a way that the distance between the object needle
and convex lens is slightly greater than the focal length of the convex lens (as measured in step 1).
4. Continually adjust the position of the convex mirror until the light rays refl ected back from the mirror

pass through the lens and form a real and inverted image coinciding with the object needle. Remove the
parallax between the image of object needle and object needle. Note the position of uprights holding
object needle, lens, and mirror.
5. Remove the mirror M without disturbing the position of object needle and convex lens. Take another
needle I' and place it on the other side of the lens as shown in Fig. 2.1(b). Adjust the position of the
needle I' so that there is no parallax between I' and the inverted image of object needle O formed

by the lens.
6. Take four sets of observations for the diff erent positions of O and L.
7. Find the index correction between the convex mirror M and image needle I'.

Observations

1. Rough focal length of the convex lens = _____ cm

2. Length of the index needle = x = _____ cm
3. Observed distance between mirror M and needle I' = y = _____ cm

4. Index correction between M and I' = (x – y) = _____ cm

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