question_answer 19)

A screen is placed 90 cm from an object. Theimage of the object on the screen is formed byconvex lens at two different locationsseparated by 20 cm. Determine the focal lengthof the lens. 

Answer:

The lens formula is                                 ...(1) We know that by convention u is negative                                                     ?(2) from eqn. (1), or                                      ?(3) Equation (3) is quadratic in u and gives two valuta of u say and  in terms of f. sum of roots                       ?(4) And product of roots Also                          ?(5) (given) Adding (4) and (5), we get, and subtracting (4) and (5), or Now or Shortcut. Usingwe get