UPSC Prelims 2011 · GS Paper I
Question 27: Science and Technology
A layer in the Earth's atmosphere called Ionosphere facilitates radio communication. Why ?
- 1.The presence of ozone causes the reflection of radio waves to Earth.
- 2.Radio waves have a very long wavelength.
Which of the statements given above is/are correct ?
AnswerThe answer is (d).
The ionosphere is a layer of the upper atmosphere, from about 60 to 1,000 km, where solar radiation ionises gases and creates free electrons and ions. These charged particles reflect radio waves of certain frequencies back to Earth, so signals can travel beyond the horizon. Ozone is found lower down, in the stratosphere, and plays no part in reflecting radio waves, and long wavelength by itself is not the reason. So neither statement gives the correct reason.
- 1. Ozone lies in the stratosphere and does not reflect radio waves.
- 2. Wavelength is not the reason; the ionised layer reflects the waves.
How you could have got this
- If you were sure statement 1 is not correct, (a) and (c) go: each of them includes 1.
What you needed to know: Ozone lies in the stratosphere and does not reflect radio waves.
- Then, if you were sure statement 2 is not correct, (b) goes: it includes 2.
What you needed to know: Wavelength is not the reason; the ionised layer reflects the waves.
Only (d) is left. This one needs every statement.
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) Ozone does not reflect radio waves; free electrons and ions in the ionosphere do.
- (b) Wavelength alone is not why the ionosphere helps radio communication.
- (c) Neither statement gives the reason, so 'both' fails.
- (d) Right: neither statement is the reason; ionised layers reflect radio waves.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2011, General Studies Paper I, Series A. The paper on upsc.gov.in.