UPSC Prelims 2013 · GS Paper I
Question 90: Science and Technology
Rainbow is produced when sunlight falls on drops of rain. Which of the following physical phenomena are responsible for this?
- 1.Dispersion
- 2.Refraction
- 3.Internal reflection
Select the correct answer using the codes given below.
AnswerUPSC’s answer is (d).
A rainbow forms when sunlight enters a raindrop and is refracted, bending as it passes from air into water. The light is split into colours by dispersion, since each colour bends by a different amount. It is then reflected inside the drop by total internal reflection from the back surface, and refracted again as it leaves. All three phenomena are involved.
- 1. Dispersion splits white light into colours.
- 2. Light refracts on entering and leaving the drop.
- 3. Light reflects internally at the back of the drop.
How you could have got this
- If you were sure statement 1 is correct, (b) goes: it leaves 1 out.
What you needed to know: Dispersion splits white light into colours.
- Then, if you were sure statement 2 is correct, (c) goes: it leaves 2 out.
What you needed to know: Light refracts on entering and leaving the drop.
- Then, if you were sure statement 3 is correct, (a) goes: it leaves 3 out.
What you needed to know: Light reflects internally at the back of the drop.
Only (d) is left. This one needs every statement.
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. Internal reflection is also involved.
- (b) Wrong. Dispersion is also involved.
- (c) Wrong. Refraction is also involved.
- (d) Correct. Dispersion, refraction and internal reflection all play a part.
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.