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UPSC 2000: 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.
(a)Both A and R are true, and R is the correct explanation of A
(b)Both A and R are true, but R is not a correct explanation of A
(c)A is true, but R is false
(d)A is false, but R is true
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Not attemptedCorrect Answer: (a) Both A and R are true, and R is the correct explanation of A
Road‐sign beads act as tiny retro-reflectors: light entering the glass strikes the curved back surface at an incidence angle greater than the critical angle, undergoes total internal reflection and is returned toward the source, making the beads appear bright. Thus both A and R are true and R correctly explains the phenomenon described in A. ([focusias.in](https://focusias.in/upsc-general-studies-prelims-paper-2000/))
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Frequently Asked Questions
What is the correct answer to this UPSC 2000 Science & Technology question?▼
The correct answer is option (a): Both A and R are true, and R is the correct explanation of A. Watch the detailed video explanation in our Sherlocking Prelims course on UnlockIAS.
Which year was this Physics question asked in UPSC Prelims?▼
This question was asked in UPSC CSE Prelims 2000, GS Paper 1, as Question 105. It falls under Science & Technology > Physics.
What is the difficulty level of this UPSC question?▼
This question is rated "moderate". It requires sound conceptual understanding of the topic.
How many UPSC Prelims questions have been asked from Physics?▼
UPSC has asked 104 questions from Physics (under Science & Technology) across multiple years. Visit the topic page to see the full list and year-wise trends.