If you could see the sight blade from the shooters' position, you would see there is a round glass vial that has been inserted into a hole made for it, sort of like a 'gold bead' blade, only the vial is recessed completely below the striations on the rear face of the sight. The vial holds, among other things, a small amount of a radioactive isotope called 'tritium' that makes the vial glow like it has a small light source inside.
Trijicon is one of the industry leaders in tritium night sights for many kinds of guns.