During an early phase of the planet's existence, earth's atmosphere was similar in composition to the atmospheres of mars and venus. at that time earth's principal atmospheric gas was

The answer is carbon dioxide. This primordial earths’ atmosphere was composed by gasses from degassing of the earth's interior after its formation. It is after the beginning of life that oxygen levels began to rise and levels of carbon dioxide began to reduce in the atmosphere (as a result of photosynthesis).

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.