UPSC Prelims 2013 · GS Paper I
Question 89: Science and Technology
Consider the following phenomena :
- 1.Size of the sun at dusk
- 2.Colour of the sun at dawn
- 3.Moon being visible at dawn
- 4.Twinkle of stars in the sky
- 5.Polestar being visible in the sky
Which of the above are optical illusions?
AnswerUPSC’s answer is (c).
The sun looks larger at dusk, its colour looks reddish at dawn, and stars appear to twinkle, because the atmosphere refracts and scatters light so that things look different from what they are. UPSC treats these as optical illusions. The moon being visible at dawn and the Pole Star being visible in the sky are real: the objects are actually there to be seen. So 1, 2 and 4.
- 1. The sun looks larger at dusk than it is.
- 2. The sun's colour at dawn is an appearance caused by the atmosphere.
- 3. The moon is really there and visible at dawn.
- 4. Stars seem to twinkle because of atmospheric refraction.
- 5. The Pole Star is really visible in the sky.
How you could have got this
- If you were sure statement 3 is not correct, (a), (b) and (d) go: each of them includes 3.
What you needed to know: The moon is really there and visible at dawn.
Only (c) is left. You didn’t need statements 1, 2, 4 or 5.
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) Wrong. The moon at dawn is real, not an illusion, and twinkling is left out.
- (b) Wrong. The moon at dawn and the Pole Star are real.
- (c) Correct. Sun's size at dusk, its colour at dawn and twinkling of stars.
- (d) Wrong. The moon at dawn and the Pole Star are real, and the sun's size at dusk is left out.
Worth knowing: Strictly, the dawn colour and twinkling are physical effects of scattering and refraction rather than illusions; UPSC's key follows loose usage.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2013, General Studies Paper I, Series A. The paper on upsc.gov.in.