What is the focal length of a convex lens of focal length 30cm in contact with a concave lens of focal length 20cm? Is the system a converging or a diverging lens? Ignore thickness of the lenses.

Explanation

Here focal length of convex lens `f_1 = + 30 cm `
Focal legth of concave lens `f_2 = -20 cm`
`1/f = 1/(f_1) +1/(f_2) = 1/(30) + 1/(-20) = (2-3)/(60) = - (1)/(6)`
`therefore f = - 60 cm `
Negative sign represents that combination works as concave lens.