# The speed of light in air is 3.0 × 10⁸ms¯¹. Its speed in glass having a…

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## The speed of light in air is 3.0 × 10⁸ms¯¹. Its speed in glass having a refractive index of 1.65 is

A. 1.82 × 10⁸ms¯¹.

B. 3.00 × 10⁸ms¯¹.

C. 4.95 × 10⁸ms¯¹.

D. 1.65 × 10⁸ms¯¹.

A

## DETAILS…

For short cut, option A is the most reasonable answer, I will explain later.

### Now for the right answer to the above question:

1. Option A is correct.
2. Option B is incorrect. the speed of light in glass can never be the same with that in air.
3. C is incorrect. the speed of light is said to be the speed limit of the universe, so we do not expect a greater speed in glass.
4. D is not the correct answer. it is less likely to be exactly same value of 1.65. the only condition for same values is when the refractive index of a material is root 3 (or 1.73).

## KEY-POINTS…

• You should be able to quickly choose option A even without doing any calculation, this is because other options are simply not reasonable. The speed of light cannot exceed 3.0 x 10 m/s, so, the answer must be less than that, and it may not exactly be equal to a factor of the given refractive index of 1.65.
• According to Einstein’s theory of special relativity, the speed of light is the speed limit of the universe, although this is currently disputed, we do not expect any value greater than the normal speed of light.
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