Question

Two thin lenses, one of focal length + 60 cm and the other of focal length – 20 cm are put in contact, the combined focal length is,

(a) 15 cm 

(b) – 15 cm 

(c) – 30 cm 

(d) 30 cm

Answer 1

(c) – 30 cm

F= \(\frac{f_1f_2}{f_1 + f_2}\) = \(\frac{60 \times (-20)}{60-20}\) = -30cm