A dentist concave mirror has a radius of curvature of `30 cm`. How far must it be placed from a small cavity in order to get a virtual image magnified `5` times ?

Answer

`- 12 cm`

Explanation

Here, `R = -30 cm, u = ?, m = 5`
From `m = - (v)/(u) = 5, v = - 5 u`
As `(1)/(v)+(1)/(u)=(1)/(f) :. (1)/(-5 u) +(1)/(u) = (1)/(-15)`
`(4)/(5 u) = - (1)/(15), u = -12 cm`.