2002 General Studies, all questions

UPSC Prelims 2002 · General Studies

Question 41: Science and Technology

Consider the following natural phenomena:

  1. 1.Terrestrial heating
  2. 2.Reflection of light
  3. 3.Refraction of light
  4. 4.Diffraction of light Due to which of these phenomena is mirage formed?
a1 and 2
b2, 3 and 4
c1 and 3
d4 only

AnswerThe answer is (c).

A mirage forms when hot ground heats the air just above it. The air layers then differ in density and refractive index, so light from the sky or a distant object bends as it passes through them, and the image appears on the ground like a pool of water. So terrestrial heating and refraction (with total internal reflection near the horizontal) cause it. Reflection from a surface and diffraction are not the cause.

  • 1. Hot ground heats the air and creates the density gradient.
  • 2. Plain reflection from a surface is not the cause.
  • 3. Refraction bends the light through the layers of unequal density.
  • 4. Diffraction is bending around edges and plays no part.

How you could have got this

  1. If you were sure statement 1 is correct, (b) and (d) go: each of them leaves 1 out.

    What you needed to know: Hot ground heats the air and creates the density gradient.

  2. Then, if you were sure statement 2 is not correct, (a) goes: it includes 2.

    What you needed to know: Plain reflection from a surface is not the cause.

Only (c) is left. You didn’t need statements 3 or 4.

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) Includes reflection, but a mirage is due to bending of light.
  • (b) Includes reflection and diffraction, neither of which makes a mirage.
  • (c) Right: terrestrial heating and refraction.
  • (d) Diffraction is the bending around edges and gives no mirage.

Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2002, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.