The focal length of the combination of two thin lenses in contact is 80 cm. If the focal length of one lens is -20 cm, what is the power of the other lens ?

Options

A. 2.25 D
B. 6.25 D
C. `-2.5` D
D. `-3.75` D

Answer

B

Explanation

The lens of focal length - 20 cm is a concave lens.
`(1)/(F)=(1)/(f_(1))+(1)/(f_(2)) therefore (1)/(80)= -(1)/(20)+(1)/(f_(2))`
`therefore (1)/(f_(2))=(1)/(80)+(1)/(20)=(1+4)/(80)=(5)/(80)`
`therefore f_(2)=16 cm=(16)/(100)m`
`therefore` Focal power of the second lens,
`P_(2)=(1)/(f_(2))=(100)/(16)D=6.25D`