Optical activity is the ability of a substance to rotate the plane of polarization of plane polarized light. Substances which have this ability are called optically active substance. Tartaric acid is an example of optically active liquid.
When a polarizer and analyzer are crossed, no light emerges out of the analyzer. When a quartz plate cut with its faces parallel to the active axis is introduced between the polarizer and the analyzer such that light falls normally upon the quartz, the light emerges out of the analyzer.
Here, the quartz plate turns the plane of vibration. This phenomenon is known as optical activity.