Question

What is the focal length of the combination of a convex lens of focal length 30 cm in contact with a concave lens of focal length 20 cm?​

Answer 1

Focal length of the convex lens, f1=30cm

Focal length of the concave les, f2=-20cm

Focal length of the system of lenses =f

The equivalent focal length of a system of two lenses in contact is given as:

1f=1f1+1f2

1f=130−120=2−360=160

∴f=−60cm

Hence, the focal length of the combination of lenses is 60 cm. The negative sign indicates that the system of lenses acts as a diverging lens.