Question

An eye specialist prescribes spectacles to a patient. The spectacles is a combination of a convex lens of focal length $20$cm and a concave lens of focal length $25$cm. The power of this lens combination will be.

• A. $2$D
• B. $-1.5$D
• C. $1$D
• D. $-1$D

Answer 1

The correct option is C $1$D

$\because p=p_1+p_2$
$=\frac{1}{f_1\left(m\right)}+\frac{1}{f_2\left(m\right)}$
$=\frac{100}{f_1\left(cm\right)}+\frac{100}{f_2\left(cm\right)}$
$=\frac{100}{20}+\frac{100}{(-25)}$
$=5-4$
$=1$D.