Two thin lens have a combined power of 10D in contact. When separated by 20 cm their equivalent power is `6.25D`. Find teir in dividual powers in dioptres-

Options

A. `3.5` and `6.5`
B. 5 and 5
C. `7.5` and `2.5`
D. 9 and 1

Answer

C

Explanation

`P = P_(1) +P_(2) rArr 10 = (1)/(f_(1)) +(1)/(f_(2))`
`P = P_(1)+P_(2) - dP_(1)P_(2)`
`rArr 6.25 =(1)/(f_(1)) +(1)/(f_(2)) -(20)/(f_(1)f_(2))`
On solving (3) option is obtained