The propagation velocity v = (ε₀ μ₀)^-0.5 characterizes electromagnetic waves traveling unimpededly like they would do so within vacuums where there's no net charge or current flow present. When passing non-conducting materials with dielectric constants K ('dielectrics'), their velocities change to v = (K ε₀ μ₀)^-0.5. The dielectric constant for glass under frequencies corresponding to the visible spectrum (~500 THz) is roughly 5. Calculate the speed of light in glass and express this as a percentage relative to its speed in a vacuum.