Light from a point source in air falls on a convex curved surface...

$\dfrac{{\mu}_{2}}{v} - \dfrac{{\mu}_{1}}{u} = \dfrac{{\mu}_{2} - {\mu}_{1}}{R}$

$⇒ \dfrac{1 .5}{v} - \dfrac{1}{- 100} = \dfrac{1 .5 - 1}{20}$

$⇒ v = 100cm$

Distance from object

$= 100 + 100$

$= 200cm$