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.

Correct Option (A)

Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).

The phenomenon described in Assertion (A), where small glass beads on traffic signals glow brightly when illuminated, is due to total internal reflection. These glass beads are designed to act as retroreflectors. When light from a vehicle's headlights enters a glass bead, it travels from a rarer medium (air) to a denser medium (glass). If the angle of incidence at the glass-air interface (the back surface of the bead) exceeds the critical angle for glass, the light undergoes total internal reflection. This reflected light then travels back towards the source, making the traffic signal appear to glow brightly.

Reason (R) accurately defines the conditions for total internal reflection:

• Light must travel from a denser medium to a rarer medium.
• The angle of incidence must exceed a specific critical value.

This principle directly explains why the glass beads exhibit their reflective property.

Incorrect Options:

• Option (B) is incorrect because Reason (R) directly explains the mechanism behind the bright glow of the glass beads, making it a correct explanation of Assertion (A).
• Option (C) is incorrect because both Assertion (A) and Reason (R) are factually correct statements. The observation of glowing glass beads is true, and the definition of total internal reflection is also true.
• Option (D) is incorrect because Assertion (A) is a true observation. Small glass beads on traffic signals do indeed glow brightly due to retroreflection, which is based on the principle of total internal reflection.