Question

At what distance from a concave lens of focal length 20cm a 6 cm tall object be placed so as to obtain its image at 15 cm from the lens?also calculate the size of the image formed. Draw a ray diagram to justify your answer for the above situation.

Answer 1

v =15
f = -20(in concve lens value of f is negative)
u =?
h =6
lens formula = 1/v - 1/u = 1/f
1/15 -1/u = -1/20
1/u = -1/20 - 1/15
1/u = -3 - 4/60
1/u = -7/60
u= -60/7cm
m=h'/h = v/u
h'/6 = 15×7/60
60h' = 630
h'. =630/60
h' = 10.5