The speed of light is minimum in:

Question

Mass numbers of two nuclei are in the ratio of 4:3. Their nuclear densities will be in the ratio of:

Answer 1

We rank the four media by refractive index, since a higher refractive index always means a lower speed of light.

Vacuum gives light its maximum possible speed, with refractive index exactly 1, so it cannot be the medium of minimum speed.
Air has refractive index very close to 1 (about 1.0003), so light speed in air is nearly the same as in vacuum, still much faster than in water or glass.
Water has a refractive index of about 1.33, light does slow down here, but less than it does in glass.
Glass has a refractive index of about 1.5, the highest of the four, and using $v = c/n$, this highest n gives the lowest speed among the options.

So light travels slowest in glass, making option D correct.

Answer 2

We rank the four media by refractive index, since a higher refractive index always means a lower speed of light.

Vacuum gives light its maximum possible speed, with refractive index exactly 1, so it cannot be the medium of minimum speed.
Air has refractive index very close to 1 (about 1.0003), so light speed in air is nearly the same as in vacuum, still much faster than in water or glass.
Water has a refractive index of about 1.33, light does slow down here, but less than it does in glass.
Glass has a refractive index of about 1.5, the highest of the four, and using $v = c/n$, this highest n gives the lowest speed among the options.

So light travels slowest in glass, making option D correct.

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