Question

The power of a diverging lens is 10D and that of a converging lens is 6D. When these two lenses are placed in contact with each other. The power of their combination will be

• A. +16D
• B. -16D
• C. -4D
• D. +4D

Answer 1

Answer: (C)

-4D

The focal length of diverging lens is

$F_1=\frac{1}{P}=-\frac{1}{10}$

The focal length of converging lens is

$F_2=\frac{1}{P}=+\frac{1}{6}$
The focal length of combined lens is

$\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}$

$P=\frac{1}{f}=-10+6=-4D$

The combined lens will act as diverging (concave ) lens.

Option C is correct.