An eye specialist prescribes spectacles having combination of convex lens of focal length 40 cm in contact with a concave lens of focal length 25 cm. The power of this lens combination in diopters is

Options

A. `+1.5 D`
B. `-1.5 D`
C. `+6.67 D`
D. `-6.67 D`

Answer

B

Explanation

Power of the lens combination is
`P=P_1+P_2=1/(f_1(in m))+1/(f_2(in m))`
`1/(+0.4 m)+1/(-0.25 m)`
`=-1.5 m^(-1)=-1.5 D`