Question

To find the focal length of convex lens
partical experiment
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Answer 1

Answer:

Aim

To find the focal length of a convex mirror, using a convex lens.

Apparatus

An optical bench with four uprights (two fixed uprights in middle, two outer uprights with lateral movement), convex lens (20 cm focal length), convex mirror, a lens holder, a mirror holder, two optical needles, (one thin, one thick) a knitting needle, and a half metre scale.

A Short Description about the Arrangement

As a convex mirror always forms a virtual image, its focal length cannot be found directly as for a concave mirror. For this purpose, indirect method is used, as described below.

An auxiliary convex lens L is introduced between the convex mirror M and object needle O as shown in ray diagram (a). Keeping the object needle at distance about 1.5 times rough focal length of convex lens, the position of convex mirror behind convex lens is so adjusted that a real and inverted image of object needle O, is formed at O itself. Under such condition, 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 mirror.

To locate the position of C, convex mirror is removed (without disturbing the object needle O and convex lens L). An image needle I is put behind the convex lens and moved to a position at which there is no parallax between tip of inverted image of O needle and tip of I needle. Position of image needle I gives position of centre of curvature C of the mirror M ray diagram (b)]

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Theory

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Ray Diagram

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Procedure

To determine rough focal length of convex lens

Follow steps 1 to 4 of Experiment 1 (Section B