
For the refraction at the left surface, u=−∞,R=+15cm.
vμ2−uμ1=Rμ2−μ1
⇒v11.5−−∞1=+151.5−1
⇒v1=45cm
For the refraction at the right surface, u=45−30=15cm,R=−15cm
vμ2−uμ1=Rμ2−μ1
⇒v21−151.5=−151−1.5
⇒v2=7.5cm
Therefore, the distance from the centre where the beam will converge is,
15+7.5=22.5cm=225mm