As compared to UV light, visible light can cure:

Although the polymerization reaction was under the direct control of the clinician, the inability of these short UV wavelengths to penetrate deeply into the resin composite meant that the incremental layers of resin could be at most 1 mm thick. Blue light can penetrate deeper into the resin than UV or violet light, such that some “blue light–cured” restorations can now be adequately light cured in increments that are up to 5-mm thick. Phillips Dental materials 12th edition pg no 174