Two lenses of focal lengths `20 cm and - 40 cm` are held in contact. The image of an object at infinity will be formed by the combination at

Options

A. `10 cm`
B. `20 cm`
C. 40 cm
D. infinity

Answer

C

Explanation

Equivalent focal length is given by,
`(1)/(f)=(1)/(f_(1))+(1)/(f_(2))`
Here, `f_(1)` = 20 cm and `f_(2)` = -40 cm
`therefore " "(1)/(f)=(1)/(20)-(1)/(40)=(2-1)/(40)rArrf=40` cm
`because " "(1)/(f)=(1)/(v)-(1)/(u)=(1)/(v)-(1)/(oo)rArrv=f= 40` cm `" "[because u=oo]`