What is visible light communication used for?

Some of the uses of visible light communication are to display information on sign boards, Li-Fi (to provide very high internet speeds), underwater communication, communication between vehicles, etc. You can read about UPSC Mains General Studies Paper-III Strategy, Syllabus & Structure in the given link.

Further readings:

1. Topic-Wise GS 3 Questions for UPSC Mains
2. Science & Technology Notes For UPSC – General Science Notes