If any problem involving circular motion, which way does the acceleration vector point ?

the acceleration vector points towards the center of the circular path

Explanation: Acceleration is the rate of change of velocity ( is a vector quantity ) by the time interval in a motion. In uniform circular motion speed of moving object is constant but the direction of the velocity vector changes which causes an acceleration pointing to the center of the circle ( which is the path followed in circular motion ).

your answer is, orbit around the nucleus

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.