An object is object at a distance of 25cm from the pole of a convex mirror and a plane mirror is set at a distance 5 cm from convex mirror so that the virtual images formed by the two mirrors do not have any parallax. The focal length of the convex mirror is

Options

A. 37.5cm
B. `-7.5cm `
C. `-37.5cm`
D. `+7.5cm`

Answer

a.

Explanation

Clearly, the distance of I from P is 15cm.
Now, `u=-25cm, v=15cm, f=?`
`(1)/(v)+(1)/(u)=(1)/(f)` or `(1)/(f)=(1)/(-25)+(1)/(15)`
or `(1)/(f)=(-3+5)/(75)` or `f=(75)/(2)cm=37.5cm`