Q: 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 dioptre is

(A) + 1.5

(B) – 1.5

(C) + 6.67

(D) – 6.67

Ans: (B)

Sol: Focal length of combination is

$\large \frac{1}{F} = \frac{1}{f_1} + \frac{1}{f_2} $

$\large \frac{1}{F} = \frac{1}{40} + \frac{1}{-25} $

$\large F = -\frac{200}{3} cm = -\frac{2}{3} m$

Power $\large P = \frac{1}{F} = -\frac{3}{2}$

P = -1.5 D