The distance between a convex lens and a plane mirror is 10 cm. The parallel rays incident on the convex lens after reflection from the mirror forms image at the optical centre of the lens. Focal length of lens will be

Options

A. `(1//4) fold 3`
B. `(3//4)fold`
C. `(5//54)fold`
D. `(9//4)fold`

Answer

d

Explanation

We know that a phase change of `pi` oocures when the reflection takes place at the boundaruyb of denser medium ,.This is equivalent to a path difference of `lamda//2`
`therefore Total phase dffeece =pi-pi=0`
Thus,the two waves superimpose in phase
Resultant amplitude =`sqrt(I)+sqrt(I/4)=3/2sqrt(I)`
Resultant amplitude =`(3)/(2)sqrt(I))^(2)=(9)/(4)I=(9)/(4)fold`