In many areas electricity is produced by huge generators close to dames. What function does a large volume of moving water have in generating electricity?

To increase the volume of moving water, impoundments or dams are used to collect the water. An opening in the dam uses gravity to drop water down a pipe called a penstock. The moving water causes the turbine to spin, which causes magnets inside a generator to rotate and create electricity.

snell's law

refractve index = sin incidence / sin refraction