6 6 D O C U M E N T 4 3 L I G H T I N D I S P E R S I V E M E D I A , (4) where is set. Condition (3) gives x = 0. The propagation of light hence occurs along the y-axis. II. Train of waves[7] of variable direction in a nondispersive medium. We set . Then . The velocity V is, in this case, independent of the frequency . Equation (3) yields . (5) That it really does involve a ray of variable direction one can see from the follow- ing. Light that is illuminating the starting point at time t passes the origin at time . The illuminated starting points lie in the direction . So this direction changes over time . The light passing the origin at a certain time propagates rectilinearly. III. Train of waves[8] of variable direction within a dispersive medium. We again set . Here, however, we must take into account that V is dependent on . If we set , then r r0 x r0 ----ξ –= r0 x2 y2 += ∂ω ∂ξ ------ - γ = ∂α ∂ξ ------ - 0 = H ω0 γξ) + ( t r0 V ---- – 1 Vr0 -------ξ x + α + = ω 2π ------ γ t r0 V ---- – ω0 V ---------- x r0 + 0 = [p. 21] t′ t r0 V ---- –= x r0 ---- V ω0 ------t′ –= t′ t′ ∂ω ∂ξ ------ - γ = ∂α ∂ξ ------ - 0 = ω n c V --- =