Optical activity is the property of certain substances (called optically active compounds) to rotate the plane of polarized light as it passes through them. This happens because such compounds contain chiral molecules, which are molecules that cannot be superimposed on their mirror image.
Key Points about Optical Activity
- Plane-polarized light: Normal light vibrates in all directions, but when passed through a polarizer, it vibrates only in one plane (plane-polarized light).
- Effect of chiral molecules: When plane-polarized light passes through a solution of a chiral compound, the plane of vibration is rotated either to the right or to the left.
- Types of optical rotation:
- Dextrorotatory (+): Rotates light to the right (clockwise). Denoted by a “+” or prefix d-.
- Levorotatory (–): Rotates light to the left (anticlockwise). Denoted by a “–” or prefix l-.
- Measurement: The angle of rotation is measured using an instrument called a polarimeter.
- Applications: Optical activity is important in chemistry, biology, and medicine because enantiomers (mirror-image isomers) of the same compound can have very different biological effects (e.g., one form may be therapeutic, the other toxic).
In simple words: Optical activity is the ability of a substance to twist the direction of polarized light because of the special 3D arrangement of its atoms.