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.

Assertion (A) is true: small glass beads on traffic signs and road markings act as retroreflectors, returning incident headlight beams toward the source and thus appearing to glow to the driver [1]. Reason (R) is also true: total internal reflection (TIR) occurs when light in an optically denser medium meets a boundary with a rarer medium at angles greater than the critical angle and is completely reflected. More importantly, R correctly explains A because the retroreflective action of glass beads involves light refracting into the bead, being internally reflected at the rear surface (often by TIR or a reflective backing) and refracting back out toward the source — the exact mechanism described by the combination of refraction and TIR [2].

Sources

1. [1] https://mkp.gem.gov.in/catalog_data/catalog_support_document/buyer_documents/8778051/54/78/703/CatalogAttrs/SpecificationDocument/2025/6/3/field_guide_glass_beads_2025-06-03-12-30-16_2c148fd56c84ba12cea5371dfc5addd9.pdf
2. [2] https://www.rema.org.uk/pub/pdf/history-retroreflective-materials.pdf