Final answer:
The strength of the permanent waving solution is mainly determined by the concentration of the reducing agent, typically ammonium thioglycolate, which breaks and then reforms the disulfide bonds in hair.
Step-by-step explanation:
The strength of the permanent waving solution is determined primarily by the concentration of the reducing agent, usually ammonium thioglycolate. This chemical is responsible for breaking some of the disulfide bonds in the hair's keratin structure, which allows the hair to be reshaped into curls. To form these curls, the solution reduces the disulfide bonds that hold the alpha-helices of the hair strands together. After the hair is curled and the solution is rinsed out, a neutralizer is then applied to help reform those disulfide bonds and lock the hair into its new shape.