Question

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

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

Answer 1

The correct option is B -1.5 D

focal length of convex lens, $f_1=+40cm$

focal length of concave lens, $f_2=-25cm$
And, as the lenses are in contact so separation between them is, $d=0$
Power of the lens can be given as,
$Power=\frac{100}{f\left(incm\right)}$
Power of the combination can be given as:
$P_{net}=P_1+P_2-d{P}_1{P}_2$

$\Rightarrow P_{net}=\frac{100}{40}-\frac{100}{25}-0$
$\Rightarrow 2.5-4$
$\Rightarrow -1.5D$
So, the correct option is $\left(B\right)$