A convex lens of focal length 80 cm and a concave lens of focal length 50 cm are combined toghether. What will be their resulting power ?

Options

A. `+6.5 D`
B. `-6.5D`
C. `+7.5 D`
D. `-0.75 D`

Answer

D

Explanation

`f_(1)=80 cm therefore " Power "P_(1) =(100)/(80)=(5)/(4)D=1.25D`
and `f_(2)=-50` cm for a concave lens
`therefore P_(2)= -(100)/(50)= -2D`
When they are combined, their resultant force length is given by `(1)/(F)=(1)/(f_(1))+(1)/(f_(2))`
or Resultant power,
`P=P_(1)+P_(2)=1.25D-2D=-0.75D`