Assertion (A): Small glass beads fixed on traffic signals glow brightly when light falls upon them. Reason (R): Light is totally reflected when the angle of incidence exceeds a certain critical value and light travelling in a denser medium is reflected from a rarer medium.

The assertion is true because glass beads used in traffic signs act as retroreflectors, causing them to appear very bright when illuminated by headlights. These beads are designed so that light enters the glass, which is a denser medium, and strikes the back surface. The reason is also true and correctly explains the assertion. It describes the phenomenon of Total Internal Reflection. For this to occur, light must travel from a denser medium like glass to a rarer medium like air at an angle of incidence greater than the critical angle. In the case of glass beads, the light undergoes internal reflection and is directed back toward the source, making the signal glow brightly to the driver. Since the reason provides the physical principle behind how the beads reflect light so effectively, option A is correct.