Any attenuating medium must exhibit this phenomenon to preserve causality. The effective mass is proportional to the curvature of functions named for this phenomenon that correspond to the bands in band diagrams. The real part of the dielectric function sharply decreases near strong absorption bands in the “anomalous” form of this phenomenon. A (15[1])“modal” form of this phenomenon spreads out signals in time in multimode (*) optical fibers. Curves of omega as a function of k are called this phenomenon’s namesake “relations.” The phase velocity for a wave is non-constant in this phenomenon, causing different frequencies to travel at different speeds. For 10 points, name this phenomenon (10[1])in which a wave’s refractive index depends on frequency, exemplified by prisms splitting white light into a rainbow. ■END■