An electric field and a magnetic field. If light were to slow down, the changing electric field would cause a diminishing magnetic field, which in turn, would produce a diminishing electric field until the wave would die out. This would be a violation of the conservation of energy. Similarly, if light would speed up, a stronger electric field would produce a stronger magnetic field, and so on. This too, would violate the conservation of energy.
Visible light is a small portion of the electromagnetic spectrum.
A radio wave is electromagnetic radiation with a frequency lower that that of light. It is of the same frequency . The two quantities are inversely proportional to the other. The greater the frequency, the shorter the wavelength. The sound coming from one tuning fork can force another to vibrate.