Optical activity is exhibited by a molecular which is contains two chiral center
Answers
The cis/trans or E/Z isomers formed by alkenes aren't the only example of stereoisomers. To understand the second example of stereoisomers, it might be useful to start by considering a pair of hands. For all practical purposes, they contain the same "substituents" four fingers and one thumb on each hand. If you clap them together, you will find even more similarities between the two hands. The thumbs are attached at about the same point on the hand; significantly below the point where the fingers start. The second fingers on both hands are usually the longest, then the third fingers, then the first fingers, and finally the "little" fingers.
In spite of their many similarities, there is a fundamental difference between a pair of hands that can be observed by trying to place your right hand into a left-hand glove. Your hands have two important properties: (1) each hand is the mirror image of the other, and (2) these mirror images are not superimposable. The mirror image of the left hand looks like the right hand, and vice versa, as shown in the figure below.
mirror
Objects that possess a similar handedness are said to be chiral (literally, "handed"). Those that do not are said to be achiral. Gloves are chiral. (It is difficult, if not impossible, to place a right-hand glove on your left hand or a left-hand glove on your right hand.) Mittens, however, are often achiral. (Either mitten can fit on either hand.) Feet and shoes are both chiral, but socks are not.
In 1874 Jacobus van't Hoff and Joseph Le Bel recognized that a compound that contains a single tetrahedral carbon atom with four different substituents could exist in two forms that were mirror images of each other. Consider the CHFClBr molecule, for example, which contains four different substituents on a tetrahedral carbon atom. The figure below shows one possible arrangement of these substituents and the mirror image of this structure. By convention, solid lines are used to represent bonds that lie in the plane of the paper. Wedges are used for bonds that come out of the plane of the paper toward the viewer; dashed lines describe bonds that go behind the paper.