Origin of optical activity
Pagni, R. M. Department of Chemistry, University of Tennessee, Knoxville, Tennessee.
- Optical rotation
- Asymmetric synthesis in nature
- Links to Primary Literature
- Additional Readings
In order to explain optical activity, it is helpful first to discuss linearly and circularly polarized light. Light can be thought of as a wave or a particle (wave-particle duality). In wave theory, light is an electromagnetic wave propagating with a velocity v (c = v in vacuum). The wave consists of an electric field E oscillating in a plane perpendicular to the direction of propagation, and a mutually perpendicular magnetic field H. Both are oscillating with a frequency ν, and the distance between successive maxima of E or H is the wavelength λ, such that λ = v/ν. If E oscillates in a specific plane relative to the propagation direction (plane of polarization), the light is said to be linearly polarized. If two mutually perpendicular linearly polarized electromagnetic waves are combined, having the same frequency and direction of propagation but with phases differing by one-quarter wavelength, the resultant E vector rotates about the propagation direction. If the direction of rotation is clockwise as viewed in the direction of the light source, the light is said to be right-circularly polarized (RCPL), and it is left-circularly polarized (LCPL) if the rotation is in the opposite direction. In particle theory, a quantum of light is called a photon, with energy equal to Planck's constant h times the frequency, or hν. Photons also have unit intrinsic angular momentum of + (equal to h/2π) along the direction of propagation (RCPL) or − against the direction of propagation (LCPL). Linearly polarized light is an equal mixture of these two photon beams.
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