Final answer:
The term that refers to the directional wave patterns or the illusion of motion in hair is 'transverse wave.' This concept applies to various physical phenomena and is illustrated by the process of waving hair, where disulfide bonds within the hair are altered to change its wave pattern.
Step-by-step explanation:
The term which refers to directional wave patterns or the illusion of motion in the hair is transverse wave. In a transverse wave, the particles of the medium through which the wave travels move perpendicular to the direction of the wave's propagation. This kind of wave can be observed when the hair undergoes styling to create waves or curls, and it's a concept that applies to other situations as well, such as the motion of water waves or the oscillations in a wave of light. The process of permanent and temporary waving of hair involves the disruption and reformation of disulfide bonds within the hair strands, which temporarily or permanently alters the direction and pattern of the hair's natural wave.
According to Huygens's principle, every point on a wavefront is a source of wavelets that spread out in the forward direction at the same speed as the wave itself, giving rise to the observed wave pattern. The disruption and reformation of bonds in hair during the waving process demonstrates how the medium (in this case, hair strands) can be manipulated to change the direction and appearance of the wave pattern.