SSC CGL 2024 Tier-I · 23 September 2024, Shift 2 · General Awareness · Question 4 of 100

Text as printed on a candidate's response sheet for this shift; answer per SSC's tentative key as marked on that sheet. Solution last reviewed 10 September 2026.

Where this paper and its answers come from

SSC does not publish CGL question papers or a public answer key. The questions and options here are reproduced as printed on a candidate's own response sheet for this shift — the Commission's text, unchanged. Each answer is SSC's tentative key as marked on that sheet; SSC allowed challenges to it and never publishes a final key. The solutions are written by UnlockIAS.

Who first stated the principle of refraction that postulates that every substance has a specific bending ratio – ‘refractive index’?

(a)Dennis Gabor
(b)Thomas Young
(c)David Brewster
(d)Willebrord Snellius

Answer

Why

Correct — D. Willebrord Snellius, the Dutch mathematician, stated the law of refraction in 1621: for a given pair of media the ratio sin i / sin r is constant.

That constant is the refractive index, which is what the stem calls a specific bending ratio. The relation carries his name in every optics syllabus as Snell's law.

Why the others are wrong

(a)
Dennis Gabor — Dennis Gabor invented holography and took the 1971 Nobel Prize in Physics for it. Wavefront reconstruction is a twentieth-century subject, three centuries after the law of refraction was written down.
(b)
Thomas Young — Thomas Young's claim is the double-slit experiment, which established the wave nature of light through interference. Interference and refraction are different phenomena.
(c)
David Brewster — David Brewster's name attaches to Brewster's angle, the angle of incidence at which reflected light is completely polarised, and to the kaleidoscope. Polarisation, not the bending ratio.

Concept

Refraction is the bending of light as it crosses into a medium where its speed differs. Snell's law fixes the geometry: n1 sin i = n2 sin r.

The refractive index of a medium is c / v, the speed of light in vacuum divided by its speed in that medium. A larger index means a slower medium and stronger bending.

Values worth carrying: about 1.33 for water, 1.5 for ordinary glass and 2.42 for diamond, which is why diamond shows total internal reflection so readily.

Priority here is genuinely muddy. Ibn Sahl described the same relation in a Baghdad manuscript of 984, and Descartes published it in 1637, which is why French texts call it Descartes' law. The key takes the textbook attribution.

Key facts

Snell's law states that n1 sin i = n2 sin r for light crossing between two media.
Refractive index is the speed of light in vacuum divided by its speed in the medium.
Thomas Young's double-slit experiment demonstrated the wave nature of light.
Dennis Gabor received the 1971 Nobel Prize in Physics for inventing holography.

Study next

Total internal reflection and the critical angleLens formula and the power of a lens in dioptresDispersion through a prism and the formation of a rainbow

Common traps

Pairing Brewster with refraction because his angle also concerns light meeting a surface
Reading refractive index as a property of the light rather than of the medium

Optics recurs as short recall. 11 Sep 2024, 12:30, General Awareness Q.13 asks the unit of the optical power of a lens, and 13 Sep 2024, 12:30, General Awareness Q.2 asks where a convex lens forms the image of an object placed at its focus.

Related PYQs

No directly related past PYQ was found.