If the speed of light in carbon disulfide and vacuum is X and Y respectively, then
- A.X < Y
- B.X > Y
- C.X ≥ Y
- D.X = Y
Correct answer
Explanation
The correct answer is A, X < Y. Light travels fastest in a vacuum; in any material medium, including carbon disulphide, it slows down, so its speed there (X) is less than its speed in vacuum (Y).
The absolute refractive index of a medium is n = c/v, the speed of light in vacuum divided by its speed in the medium. Because n is greater than 1 for every material medium, v is always less than c. Carbon disulphide is a dense, highly refracting liquid with a refractive index of about 1.63, so light in it moves at roughly 1.8 × 10⁸ m/s, against 3 × 10⁸ m/s in vacuum. Water (about 1.33) and ordinary glass (about 1.5) slow light less; diamond (about 2.42) slows it more.
B is wrong: no material medium lets light travel faster than in vacuum. C is wrong: X can be neither greater than nor equal to Y. D is wrong: the two speeds are equal only when the medium is vacuum itself.
Exam tip: n = c/v; the higher the refractive index (diamond 2.42, carbon disulphide 1.63, glass 1.5, water 1.33), the slower light travels.
Practice Questions
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