UPSC Prelims 2002 · General Studies
Question 69: Science and Technology
Consider the following statements:
- 1.Light of longer wavelength is scattered much more than light of shorter wavelength.
- 2.The speed of visible light in water is 0.95 times its speed in vacuum.
- 3.Radio waves are produced by rapidly oscillating electrical currents.
- 4.To detect over-speeding vehicles, police use the Doppler effect of reflected short radio waves.
Which of these statements are correct?
AnswerThe answer is (d).
Statement 3 is true: radio waves are produced by rapidly oscillating electric currents. Statement 4 is true: speed guns send short radio waves and use the Doppler shift of the reflected wave to find a vehicle's speed. Statement 1 is false: shorter wavelengths are scattered far more, which is why the sky is blue. Statement 2 is false: light travels in water at about 0.75 of its vacuum speed (refractive index about 1.33).
- 1. Shorter wavelengths scatter much more than longer ones.
- 2. Light in water travels at about 0.75 of its speed in vacuum.
- 3. Oscillating electric currents produce radio waves.
- 4. Speed guns use the Doppler shift of reflected radio waves.
How you could have got this
- If you were sure statement 1 is not correct, (a) and (b) go: each of them includes 1.
What you needed to know: Shorter wavelengths scatter much more than longer ones.
- Then, if you were sure statement 2 is not correct, (c) goes: it includes 2.
What you needed to know: Light in water travels at about 0.75 of its speed in vacuum.
Only (d) 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) Both statements 1 and 2 are false.
- (b) Statement 1 is false.
- (c) Statement 2 is false.
- (d) Right: statements 3 and 4.
Explanation: written by UPSC Grade, not by UPSC.
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.