cis isomer cannot be superimposed on its mirror image. It is therefore chiral and hence optically active. The trans isomer has a mirror plane and can be superimposed on its mirror image; it is achiral and optically inactive.

Are trans isomers optically active?

Trans isomer has no plane of symmetry and is optically inactive.

Why cis isomers show optical activity?

For example in a cordination entity of the type (PtCl2(en)2)^2+,only the cis-isomer shows optical activity due to lack of plane of symmetry.In trans isomer,the molecule will be symmetric,because of which its mirror images will be superimpossable. Thus the compound will be optically inactive.