How do optical isomers affect polarized light?

Its enantiomer, its mirror image, will rotate plane-polarized light in the opposite direction; i.e. it will rotate plane-polarized light by the angle #-alpha#.

How do I know this? Well, by experiment; mind you, these are exceedingly difficult experiments to do. Most of the time, we take stereoisomers from natural sources, which already display specific chirality, and a specific optical rotation, that is established by experiment. Should the enantiomers of these stereoisomers be artificially synthesized and isolated (and again these are exceedingly difficult and time-consuming experiments to do!), these isomers will display the opposite rotation.

Should a 50:50 mix of optical isomers be obtained (which is the normal scenario), this racemate will not rotate plane-polarized light because this mixture is not homochiral, or not optically pure.

So why do optical isomers behave in this way? Because plane polarized light has optical properties that predictably couple with homochiral isomers (well predictable on the basis of experiment). For chemists, simple folk, you don't need to know much more than this.

If you are not satisfied, I (and others) will try again.