Question

A thin convex lens of focal length 20 cm is kept in contact with a thin concave lens of focal length 15 cm. Find the focal length and the nature of the combination​

Answer 1

combined focal length's formula = {1/f}¹ + {1/f}²={1/F}

so, 1/20 + 1/15= 1/F

1/F = 60/7 cm