Question
Wed March 04, 2015 By: P S Lakshmi

the image  of an object placed 36 cm from a shpericl lens is formed on a screen at a distace of 72 cm from lens Find its f?
Will v=72 or v= -72  why

Expert Reply
Yashvanti Jain
Wed March 04, 2015
Given that:
Object distance, u = -36 cm ;
Image distance, v = 72 (image formed is real so taken as positive)
Focal length, f = ?
Substituting these values in the mirror formula we get:
begin mathsize 12px style 1 over straight f space equals space 1 over straight v space minus space 1 over straight u space space space space space space equals space 1 over 72 space minus space fraction numerator 1 over denominator left parenthesis negative 36 right parenthesis end fraction space equals space 3 over 72 1 over straight f space equals space 1 over 24 straight f space equals space 24 space cm end style
Thus, the focal length of the lens is 24 cm.
Home Work Help