An object is kept at a distance of 16 cm from a thin lens and the image formed is real. If the object is kept at a distance of 6 cm from the same lens, the image formed is virtual. If the size of the image formed are equal, the focal length of the lens will be

Answer

11 cm

Explanation

For real image m = `(-f)/(f+16)`
Virtual `m= (f)/(f+6)`
`therefore (-f)/(f+16) = (f)/(f+16) implies f+16 = -f-6`
`2f = -22 implies f = 11 cm`