Question

Two lenses of power $-15D$ and $-5D$ are in contact will each other. The focal length of the combination:

• A. $-20cm$
• B. $-10cm$
• C. $+20cm$
• D. $+10cm$

Answer 1

The correct option is A $-20cm$

Given that,

lenses power

$P_1=-15D$

$P_2=-5D$

We know that,

$P=\frac{1}{f}$

Now,

$P=P_1+P_2$

$P=-15-5$

$P=-20$

Now, the focal length is

$f=\frac{1}{P}$

$f=\frac{1}{-10}$

$f=0.02m$

$f=-20cm$

Hence the focal length is -$20cm$