UPSC Prelims 2000 · General Studies
Question 105: Science and Technology
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 back into a rarer medium.
AnswerThe answer is (a).
Tiny glass beads on traffic signals and road signs work as retro-reflectors. Light that enters a bead is refracted, strikes the back surface at an angle greater than the critical angle, and undergoes total internal reflection, so it returns towards the source and the bead seems to glow. So A is true, R is true, and R explains A.
- A. Glass beads on traffic signals glow brightly when light falls on them.
- R. Total internal reflection beyond the critical angle sends the light back; this explains A.
How you could have got this
This follows an unofficial answer: UPSC has not published its key for this year, and if that answer is wrong, so is this path.
Which options go is worked out from UPSC’s key and the options, not by a model.
Why each option is right or wrong
- (a) Right: both are true, and total internal reflection explains the glow.
- (b) R directly explains A, so it is the correct explanation.
- (c) R is true, so it cannot be called false.
- (d) A is true, so it cannot be called false.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2000, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.