The reduced speed of light in a medium is indicated by
that medium's 'index of refraction'.
The index of refraction of a substance is
(speed of light in vacuum) divided by (speed of light in that substance) .
The number is always more than 1.00 . If it were less than 1,
that would imply that the speed of light in that substance is
greater than the speed of light in vacuum, which can't happen.