Imagine you hold one end of a slinky or a coiled spring and a friend holds the other end to maintain some tension. You move your hand quickly towards your friend to create a disturbance that moves towards him along the slinky. Now imagine moving your hand quickly
perpendicular to the slinky. Finally, imagine creating a disturbance that is partly along the slinky and partly perpendicular (moving your
hand diagonally). What would happen to this disturbance if the transverse part and the longitudinal part traveled at different
speeds?
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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.