The Cotton effect is the characteristic change in optical rotatory dispersion

Optical rotatory dispersion

Optical rotatory dispersion is the variation in the optical rotation of a substance with a change in the wavelength of light. Optical rotatory dispersion can be used to find the absolute configuration of metal complexes....

 and/or circular dichroism

Circular dichroism

Circular dichroism refers to the differential absorption of left and right circularly polarized light. This phenomenon was discovered by Jean-Baptiste Biot, Augustin Fresnel, and Aimé Cotton in the first half of the 19th century. It is exhibited in the absorption bands of optically active chiral...

 in the vicinity of an absorption band

Absorption band

An absorption band is a range of wavelengths, frequencies or energies in the electromagnetic spectrum which are able to excite a particular transition in a substance...

 of a substance.
In a wavelength region where the light is absorbed, the absolute magnitude of the optical rotation

Optical rotation

Optical rotation is the turning of the plane of linearly polarized light about the direction of motion as the light travels through certain materials. It occurs in solutions of chiral molecules such as sucrose , solids with rotated crystal planes such as quartz, and spin-polarized gases of atoms...

 at first varies rapidly with wavelength, crosses zero at absorption maxima and then again varies rapidly with wavelength but in opposite direction. This phenomenon was discovered in 1895 by the French physicist Aimé Cotton

Aimé Cotton

Aimé Auguste Cotton was a French physicist known for his studies of the interaction of light with chiral molecules...

(1869–1951).

The Cotton effect is called positive if the optical rotation first increases as the wavelength decreases (as first observed by Cotton), and negative if the rotation first decreases.
Protein structure like beta sheet shows positive Cotton Effect.