An object has image thrice of its original size when kept at `8 cm` and `16 cm` from a convex lens. Focal length of the lens is

Options

A. 8 cm
B. 16 cm
C. between 8 cm and 16 cm
D. Less than 8 cm

Answer

C

Explanation

`m = pm3 `, as m = `(v)/(u) = (f)/(f + u)`
For virtual image , m = 3
`rArr (f)/(f - 8) = 3 rArr f = 3f - 24 rArr f = 12 cm `
For real image m = -3
`rArr (f)/(f-16) = -3`
`rArr f = -3 f + 48 rArr f = 12 ` cm.