UPSC Prelims 2000 · General Studies

Question 104: Science and Technology

Assertion (A): In a motion picture, usually 24 frames are projected every second over the whole length of the film. Reason (R): An image formed on the retina of eye persists for about 0.1 s after the removal of stimulus.

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

bBoth A and R are true, but R is not a correct explanation of A

cA is true, but R is false

dA is false, but R is true

AnswerThe answer is (a).

A film is a series of still pictures shown at 24 frames a second. The image on the retina lasts about a tenth of a second after the light is gone, so successive frames blend into each other and the eye sees smooth motion, not flicker. So A is true, R is true, and persistence of vision explains why the frame rate works.

A. Films are usually projected at 24 frames per second.
R. The retinal image persists for about 0.1 s, which explains why the frames merge into motion.

How you could have got this

If you were sure statement 1 is correct, (d) goes. Then judge the second statement; if it is correct too, decide whether it explains the first.

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) Right: both are true, and persistence of vision explains the frame rate.
(b) R explains A directly, so it is the correct explanation.
(c) R is true, so it cannot be called false.
(d) A is true, so it cannot be called false.

Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2000, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.