Question

A concave lense has focal length of20cm at what distance from the lense a 5cm tall object be placed so that ift forms an image at15cm from the lense? Also calculate the size of image formed

Answer 1

1/f = 1/v - 1/u

1/20 = 1/15 - 1/u

1/u = 5/15(20)

u = 60 cm

-v/u = Hi/Ho

-15/60 =Hi/5

Hi = -5/4 = -1.25 cm.   (-ve sign indicates image is inverted)