Where should an object be placed on the axis of a convex lens of focal length 8 cm, so as to achieve magnification of -4? (Distances are measured from optic centre of the lens)

Options

A. -6 cm
B. -10 cm
C. -12 cm
D. -9 cm

Answer

B

Explanation

We know that, lens formula, `(1)/(f)=(1)/(u)-(1)/(v)`
Magnification, `m=-(v)/(u)=-4`
`implies" "v=+4u`
`:." "(1)/(+8)=(1)/(-u)-(1)/(4u)`
(-u, because of sign convention)
`implies" "(5)/(4u)=-(1)/(8)impliesu=(-40)/(4)=-10" cm"`