Tutorial queshers set 1| why does molecule become a chiral if has a plane of symmetry even through it has stereocenter

Here's what I get.

Explanation:

Consider the molecule shown below.

It has two chiral centres, labelled R and S, and an internal mirror plane that makes the right and left halves of the molecule mirror images of each other.

If we pass a beam of plane-polarized light through the sample, the R portion of the molecule will rotate the light in a certain direction.

However, the S portion will rotate the light by the same amount in the opposite direction.

The two effects cancel each other, so the molecule has no net effect on plane-polarized light.

Even though the molecule has two chiral centres, it is achiral because it has an internal plane of symmetry.