refractivity

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re·frac·tiv·i·ty

(rē'frak-tiv'i-tē),
Refractive power.
Synonym(s): refringency

re·frac·tiv·i·ty

(rē'frak-tiv'i-tē)
Refractive power.

refractivity

(rē″frăk-tĭv′ĭ-tē)
The quality of being refractive; the ability to refract.

dispersion

Phenomenon of the change in velocity of propagation of radiation in a medium, as a function of its frequency, which causes a separation of the monochromatic components of a complex radiation. All optical media cause dispersion by virtue of their variation of refractive index with wavelengths. Dispersion is specified by the difference in the refractive index of the medium for two wavelengths. The difference between the blue F (486.1 nm) and the red C (656.3 nm) spectral lines is called the mean dispersion, i.e. nFnC. Dispersion is usually represented by its dispersive power ω or relative dispersion which is equal to the mean dispersion divided by the excess refractive index of the helium d (587.6 nm) spectral line (nd − 1), often called the refractivity of the material,
ω = nFnC/nd − 1
The reciprocal of the dispersive power is called the Abbé's number or constringence (Fig. D8). See aberration longitudinal chromatic; achromatic axis; Fraunhoffer's lines; achromatic prism.
Fig. D8 Dispersion of a white beam of light by a prismenlarge picture
Fig. D8 Dispersion of a white beam of light by a prism