Question

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

• A. $+1.5$
• B. $-1.5$
• C. $6.67$
• D. $-6.67$

Answer 1

The correct option is B $-1.5$

Power of the combination, $P=P_1+P_2-d{P}_1{P}_2$

$=\frac{100}{4}-\frac{100}{25}-0=2.5-4=-1.5D$