What is meant by a mass measurement expressed in this form: 4.6 g â± 0.2 g?

The given mass measurement contains some amount of uncertainty, this uncertainty is 0.2 g.  Generally, (4.6 g±0.2 g) does not say that the “true” mass is in the interval 4.4 g to 4.8 g for sure. It means that the mass was measured with a precision of 0.2 g.

Thus, the mass measurement value of 4.6 g ± 0.2 g, describe the range of anywhere from 4.4 grams (4.6 - 0.2) to 4.8 grams (4.6 + 0.2).

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.