The wavelength of light is λ = c / f, where c is the speed of light and f is the frequency. When c is approximately 10^7 times lower, so is the wavelength (if

The wavelength of light is λ = c / f, where c is the speed of light and f is the frequency. When c is approximately 10^7 times lower, so is the wavelength (if we keep the frequency the same), so now-visible light would have wavelengths far shorter than an atom or a molecule, and probably couldn't be easily detected by biological means. You could say: sure, maybe we'll just see light at lower frequency. Maybe that's true, but the energy of a single photon is E = hf, where h is the Planck constant. So if we assume much lower-frequency photons, they have much lower energy, and thus become harder to detect. Lots of other things could be different, like chemistry, heat conduction etc. At a height of roughly 1.8, at c = 5km/h, light would take 0.1 seconds to travel from head to toe. Nerve signals are much slower than c, so human bodies likely wouldn't exist as we know them. The list of potential implications to consider is fascinating, endless and maybe even a bit overwhelming :-)