Question

If two $+5D$ , lenses are mounted at some distance apart, the equivalent power will always be negative, if the distance is

• A. Greater than $40cm$
• B. Equal to $10cm$
• C. Greater than 8 $0cm$
• D. Less than $10cm$

Answer 1

Answer: (A)

When two lenses are separated by some distance $x$ , then equivalent power
$P=P₁+P₂-xP₁P₂$
$\therefore P=5+5-x\times 5\times 5$
$orP=10-25x$
Power $P$ will be negative if 10-25x will be negative
$ie,25x>10$
$orx>\frac{10}{25}$
$orx>\frac{10}{25}\times 100cm$
$orx>40cm$