Question

Two lenses of power -15 D and + 5D are in contact with each other. The focal length combination is:

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

Answer 1

The correct option is B -10 cm

Power of combination of two lenses,

$P=P_1+P_2$

$=-15+5$

$P=-10D$

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

So focal length,$F=-\frac{1}{10}m$

$F=-10cm$