Correct Answer - Light on the axis will be from 10cm to 20 cm for the lens
`(1)/(f)=(mu-1)(1/R_(1)-1/R_(2))`
`=(1.5+0.5r-1)(2)/(20)=0.5(1+r)(1)/(10)`
`(1)/(f)=(1+f)/(20)rArrf=(20)/((1+r))`
For `r=0, f=20cm`,
For `r=1cm, f=10cm`
Light on the axis will be from 10cm to 20cm for the lens.