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UPSC 1995: 112. Suppose a rocketship is receding from the earth at a speed of 2⁄10th the velocity of light. A light in the rocketship appears blue to the passengers on the ship. What colour would it appear to an observer on the earth?
(a)Blue
(b)Orange
(c)Yellow
(d)Yellow-orange
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Not attemptedCorrect Answer: (d) Yellow-orange
Because the rocket is moving away at 0.2 c, the light it emits is red-shifted. Using the relativistic Doppler formula λ′ = λ(1+v/c)/√(1−v²/c²), blue light (≈ 450–475 nm) is shifted to about 563–594 nm, which lies in the yellow-orange region of the visible spectrum. Hence the earth-based observer sees the light as yellow–orange, not blue, yellow or pure orange. ([sarthaks.com](https://www.sarthaks.com/66062/suppose-rocketship-receding-from-the-earth-speed-velocity-light-light-rocketship-appears?utm_source=openai))
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Frequently Asked Questions
What is the correct answer to this UPSC 1995 Science & Technology question?▼
The correct answer is option (d): Yellow-orange. 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 1995, GS Paper 1, as Question 112. It falls under Science & Technology > Physics.
What is the difficulty level of this UPSC question?▼
This question is rated "difficult". It is a challenging question that demands deep subject knowledge and analytical thinking.
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.