Question

If two lenses of `+5` dioptres are mounted at some distance apart, the equivalent power will always be negative if the distance is
A. Greater than `40 cm`
B. Equal to `40cm`
C. Equal to `10 cm`
D. Less than `10 cm`

Answer 1

Correct Answer - A
Using `P=P_(1)+P_(2)-d xx P_(1)P_(2)`
for equivalent power to be negative
`d xx P_(1)P_(2)gtP_(1)+P_(2)impliesd xx 25gt10`
`impliesdgt(10)/(25)impliesdgt(10xx100)/(25)impliesdgt40cm.`