Rafiq should expose one piece under the lamp for 10 days while keeping the other in the darkness.
In this experiment, it is essential to have a control group and an experimental group.
Based on the previous ideas, the best for him is to keep one piece of clothing in the darkness (control group), while exposing the other to the light of the lamp (experimental group).
In this way, he can determine if there is a real change of color by comparing both pieces after 10 days.
Learn more about control group in: https://brainly.com/question/1073731