Sunspots are cooler than the surrounding solar surface because

Because the convection process stops and hotter plasma doesn't replace the cooler plasma on the surface

Explanation:

Sun spots are the darker and relatively cooler regions on the surface of the Sun (photosphere). Their temperature is around 1000° C lower than the Sun's surface.

Reason: The Sun's surface is granulated because of the convection process happening in the layer just below the photosphere. Because of the convection, hotter plasma comes on the surface, release energy, and then gets replaced by the hotter plasma. Just like tea or water boils. The places where magnetic field line enter into and exit from the surface, this convection process stops. Thus these area starts getting cooler and appear black in color.

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.