In the UV radiation experiment, what was the purpose of exposing one plate to visible light?

I need help with this micro lab question , we did an experiment with some bacteria ,

E. Effect of UV radiation

Per table: 12 TSA plates (6/pair of students), liquid cultures of Serratia marcescens and Bacillus subtilis,
sterile cotton swabs, UV lamp, visible light source
Note: Serratia marcescens produces a red pigment known as prodigiosin especially when it is grown at room temperature. However, cells that have mutations in the genes responsible for producing this pigment may lose their ability to make it.
1. Each pair of students will work with one type of bacteria (Serratia marcescens or Bacillus subtilis). Label your plates with the name of the organism and the experimental conditions as follows:
1. UV lid off 5 sec
2. UV lid off 10 sec
3. UV lid off 1 min
4. UV lid off 10 min
5. UV lid on 10 min (control)
6. Visible light (lid off) 10 min (control)


4. In the UV radiation experiment, what was the purpose of exposing one plate to visible light?