UPSC Prelims 1995 · General Studies
Question 112: Science and Technology
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?
AnswerThe answer is (d).
Light from a receding source is shifted to longer wavelengths, the Doppler red shift. At 0.2 times the speed of light the wavelength grows by about 22 per cent, since the square root of 1.2 divided by 0.8 is about 1.22. Blue light of about 450 to 490 nm becomes roughly 550 to 600 nm, which spans yellow to orange, and the given key picks yellow-orange. Remember: a receding source shows a red shift.
Why each option is right or wrong
- (a) Blue would show only if the ship were stationary; the recession shifts it to longer wavelengths.
- (b) Orange needs a larger shift than a mid-blue gets here.
- (c) Yellow is within the range; a mid-blue gives yellow, but the key prefers yellow-orange.
- (d) The given key: yellow-orange.
Worth knowing: Disputed: the result depends on which blue is meant, and a mid-blue near 475 nm shifts to about 580 nm, which is yellow (c).
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 1995, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.