So my teacher told that those compounds which have one or more chiral carbon present in them it will definitely show optical isomerism because it has an optically active stereoisomer, but there are many compounds which do not have any chiral centre like, how will you determine which shows optical isomerism among the following.
In the first image due to the absence of any chiral carbon or steriogenic centre it does not show optical activity. In the second image there is a plane of symmetry thus it ceases to be optically active(the plane of symmetry is perpendicular to the substituted butane ring).finally Alenes are optically active as the successive π orbitals lie on a different plane.You can definitely get the structure of alene from a reference book and the due to successive π bonds they lie on a different plane (the π orbitals).thus they are optically active. Thus in compounds where there are no chiral centre check centre of symmetry,axis of symmetry and plane of symmetry as a CHIRAL carbon is essentially an unsymmetrical molecule.Hope you understood:).
Optical isomerism is exhibited by all species that rotate ppl in either clockwise or anticlockwise direction. This is checked by fact ( unsymmetrical molecules rotate ppl in either direction). Unsymmetry of molecules can be due to absence of one or more than 1 amongst (1 ) Chiral centre( 2 )Centre of symmetry (3) axis of symmetry(4) alternate axis of symmetry. In first question there is plane of symmetry and absence of chiral centre therefore molecule is symmetric thus not shows optical isomerism. In 2,there is absence of all 4,optically inactive. In 3 there is plane of symmetry as both Cl are same side. If both were at opposite side then also they have point of symmetry, optically inactive in both cases mentioned.