Q. 47

# The convex surfac

(a) Let F be the focal length of the given concavo-convex lens.

Then, --------------- (i) The value of can be obtained by using lens makers’ formula:   Substituting these values in equation (i): F= For the image to be informed at the same point as the object

u = 2F = 2 x 7.5 = 15 cm.

Therefore, the pin should be placed at a distance of 15 cm from the lens.

(b) If the concave part is filled with water;

If is the focal length of lens in water, the focal length F’ of this combination is given by:  ----------------------- (ii) The value of is calculated by Lens makers’ formula:   Substituting these values in equation (ii):  u’ = 2F= 2× = cm

Displacement of pin = u-u’ = (towards the lens).

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