(a) List four characteristics of the image formed by a convex lens when an object is placed between its optical centre and principal focus. (b) Size of the image of an object by a concave lens of focal length 20 cm is observed to be reduced to (1)/(3) rd of its size. Find the distance of the object from the lens.

(a) List four characteristics of the image formed by a convex lens whe

1.	(a) List four characteristics of the image formed by a convex lens when an object is placed between its optical centre and principal focus. (b) Size of the image of an object by a concave lens of focal length 20 cm is observed to be reduced to (1)/(3) rd of its size. Find the distance of the object from the lens.
Answer» SOLUTION :(a) When an object is placed between the optical centre and PRINCIPAL focus of a convex lens, the image formed is virtual, erect and enlarged. Moreover, the image is formed on the same side of lens behind the object. (b) Here magnification of given concave lens `m = + (1)/(3)` and focal length of lens f = - 20 cm As per relation `m = (v)/(u)` for a lens, we get `+ (1)/(3) = (v)/(u) RARR v = + (u)/(3)` Therefore, as per sign convention followed, both u and v are -ve. Using lens formula `(1)/(v) - (1)/(u) = (1)/(f)`, we have `(1)/((-(u)/(3))) - (1)/((-u)) = (1)/((-20)) rArr (-3)/(u) + (1)/(u) = - (1)/(20) rArr - (2)/(u) = (-1)/(20) rArr u = 40` cm So the object is placed at a distance of 40 cm from the lens.