Question

does the speed of light increase or decrease in a medium in comparison of its value and vacuum

Answer 1

speed of light is fastest in vaccum compared to al! other! mediums.... I.e 3*10^8m/s

Answer 2

Yes speed of light decreases in value in other mediums compared to vacuum.

In vacuum the speeed of light is highest.

It is $3*10^8m/s$ approximately.

Hope it helps you.

Thanks.