text: This quantity equals one plus the quantity c over pi, times the Cauchy principal value of an integral whose numerator contains the absorption coefficient.
full_question: This quantity equals one plus the quantity c over pi, times the Cauchy principal value of an integral whose numerator contains the absorption coefficient. This quantity measured at the D, F, and C Fraunhofer lines is used to calculate the Abbe number. In anomalous dispersion, this quantity decreases as frequency increases. Neglecting any imaginary component, the real part of this quantity is the square root of the dielectric constant. This quantity multiplies sines and cosines of incident angles in the Fresnel equations, and a ratio of it is used to define the critical angle. It equals the velocity of light in a medium over the speed of light. For 10 points, name this quantity equal to 1.67 for glass and symbolized n.
first_sentence: This quantity equals one plus the quantity c over pi, times the Cauchy principal value of an integral whose numerator contains the absorption coefficient.
raw_answer: index of refraction [or refractive index; or n until it is read] 