Question

How far should an object be placed from a convex lens of focal length 20 cm to obtain its real image

at a distance of 30 cm from the lens? Determine the height of the image if the object is 4 cm tall.​

Answer 1

Answer:

Formula for image- object relationship:

                                                                   1/u+1/v=1/f

                               u is the object distance

                                v is the image distance

                                 f is the focal length of the lens

If you know the focal length and object distance, using the above formula image distance can be calculated.

        1/u + 1/30 = 1/20

         1/u =  1/20 - 1/30

         1/u = 1/60

           u = 60 cm  so first ans is 60cm object should be placed at 60 cm from lens    

2nd part calculation for image height:

M= (image height)/(object height) = - (image distance)/(object distance)

        I/O = -ve(30/60)

         I/6 = -ve(1/2)

         I=  -3cm  here -ve sign shows that image in inverted ANS

Explanation: