What effect does a cold wave have on hair?

A cold wave, in the context of hair treatments, refers to a perm technique that utilizes a cold solution to alter the hair's structure, creating waves or curls. The correct answer highlights that a cold wave softens and expands the hair strand. This is because the cold perm solution works by breaking down the disulfide bonds within the hair's keratin structure, allowing it to take on a new shape without using heat.

As the chemical solution penetrates the hair shaft, it causes the hair fibers to swell, leading to an expansion of the hair strand. This softening is essential for the hair to accept the desired curl or wave shape when wrapped around the perm rods.

Choosing the other options would not align with the scientific processes occurring during a cold wave treatment. The process does not inherently strengthen the hair strand, does not lighten the hair color, nor does it straighten the hair, which distinguishes the cold wave's purpose and outcome as primarily focusing on creating waves through softening and expanding the hair.