You Do Need power plug adapter while traveling from Namibia to Sri Lanka.

Here is Why You Need An Adapter

The outlet and plug type used in Sri Lanka is G and in Namibia are D and M. This is how they look like.

Namibia

In Namibia, outlets of type D and M are in use.

Type D

Type M

Sri Lanka

In Sri Lanka, outlets of type G are in use.

Type G