When a chare `+Q` is placed at the center of spherical cavity, shown in Fig, charge induced on the inner surface of shell `= -Q` and charge induced on the outer surface of the shell `= +Q`.
`:.` Surface charge density on the inner surface `= (-Q)/(4pi R_(1)^(2))`, and
Surface charge density on the outer surface `= (+Q)/(4pi R_(2)^(2))`
![image](https://learnqa.s3.ap-south-1.amazonaws.com/images/16111142227536109401611114222.png)