In millikan type appratus, the horizontal plate are 1.5 cm apart. with the electric field switched off an oil drop is observed to fall with the steady velocity 2.5×10^-2 cm/s . When the electric field is switched on the upper plate being positive, the drop just remains stationary when the p. d between plates is 1500v. Calculate the radius of the drop and how many electronic changes does it carry?

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.

im srry but ur wrong the answer is d a smooth rock, some red dirt, and an empty bird's nest.