Wave fronts can be very useful in picturing how the phase of a wave may change as it reflects or as it refracts. All points on a wave front are points of equal amplitude (equal disturbance) that move in space in unison and undisturbed, unless different portions of the wave front go through regions of space that have different properties. In that case the wave front distorts its shape, so that still all points on it retain their "equal amplitude" condition.