UPSC Prelims 2008 · General Studies

Question 133: Science and Technology

Assertion (A): In the visible spectrum of light, red light is more energetic than green light. Reason (R): The wavelength of red light is more than that of green light.

aBoth A and R are individually true and R is the correct explanation of A

bBoth A and R are individually true but R is not the correct explanation of A

cA is true but R is false

dA is false but R is true

AnswerThe answer is (d).

The assertion is false. The energy of light of a given colour is E = hc divided by the wavelength, so a shorter wavelength carries more energy. Green light has a shorter wavelength than red, so green is the more energetic of the two. The reason is true: red light does have a longer wavelength than green. Violet has the most energy in the visible spectrum and red the least.

A. Red light is less energetic than green because its wavelength is longer.
R. Red light has a longer wavelength than green.

How you could have got this

If you were sure statement 1 is not correct, only (d) is left: every other option says it is correct.

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) A is false because red is less energetic than green.
(b) A is false, so both cannot be individually true.
(c) A is false, so it cannot be true.
(d) A is false and R is true.

Explanation: written by UPSC Grade, not by UPSC.

Practise this paper timedPrevious question Next question

Source: UPSC, Civil Services (Preliminary) Examination 2008, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.