Amplitude of a signal measures the maximum deviation in magnitude from mean position. Frequency of signal measures the number of occurrences of signal (cycles) in unit time (1s).
The velocity of light depends on refractive index of the medium inversely, denser is the medium, higher is the refractive index and slower is the velocity of light and vice versa.
Any quantity whose value does not change with time is called as constant. In physics there are 2 types of constants : Dimensional and dimensionless.