33 Optical activity
33.1 Optical Activity
Optical activity describes the phenomenon by which chiral molecules are observed to rotate
polarized light in either a clockwise or counterclockwise direction. This rotation is a result of
the properties inherent in the interaction between light and the individual molecules through
which it passes. Material that is either achiral or equal mixtures of each chiral configuration
(called a racemic mixture) do not rotate polarized light, but when a majority of a
substance has a certain chiral configuration the plane can be rotated in either direction.
33.2 What Is Plane Polarized Light?
Polarized light consists of waves of electromagnetic energy in the visible light spectrum
where all of the waves are oscillating in the same direction simultaneously. Put simply,
imagine a ray of light as a water wave, with crests and peaks. All the peaks of a water
wave point in the same direction (up, against gravity) pretty much at the same time. Light
is not usually this way - its peaks and troughs are often in random array, so one ray of
light’s peaks might point in a direction 90oopposite of another ray. When all of the rays
have their peaks pointing in the same direction - like all the waves in the ocean have peaks
pointing up - then those rays of light are said to be polarized to one another.