Question

Two lenses of power + 5 D each separated by a distance will become sun-goggles when the value of d is:

• A. zero
• B. 10 cm
• C. 25 cm
• D. 40 cm

Answer 1

The correct option is C 25 cm

For combination of two lenses,

$\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}-\frac{d}{f_1{f}_2}$

$10=10-\frac{d}{25}\because power=\frac{1}{focallength}$

$d=25cm$