Optical activity is the ability of certain substances to rotate the plane of polarized light. This phenomenon occurs when light passes through chiral molecules, which are molecules that cannot be superimposed on their mirror images. Common examples of optically active substances include sugars and amino acids. The degree of rotation depends on several factors, including the type of substance, its concentration, and the wavelength of light used. Optical activity is an important property in fields like chemistry and pharmacology, as it helps in identifying and characterizing chiral compounds, which are crucial in the development of many medications.