Later in the 18th century people made discoveries in electricity and magnetism. Over 300 years researchers have made measurements of c and refined it to the point that now the SI unit of distance the meter is based on it. Light travels at approximately 300000 kilometers per second in a vacuum which has a refractive index of 10 but it slows down to 225000 kilometers per second in water refractive index of 13.
The maximum speed of light and thus the speed light travels unimpeded is constant.
Quick video on how to calculate for either the frequency or wavelength of light using the speed of light equation. Later in the 18th century people made discoveries in electricity and magnetism. No seriously we dont measure the speed of light which always refers to the speed in a vacuum. Einstein Equations Formulas Calculator Physics - Nuclear and Atomic Energy Note.