What kind of light is actually measured? Visible light is only one part of the family of light.
Light is a form of electromagnetic radiation. Other forms include radio waves, x-rays, infra-red, ultra-violet, etc. All electromagnetic radiation travels at the speed of light (i.e., they all travel at the same speed) which in free space (i.e., vacuum) is usually denoted by the letter 'c' and has a value of roughly 3x10^8 meters per second. The various form of electromagnetic radiation listed above correspond to various parts of the electromagnetic spectrum, meaning that they have different frequencies, or equivalently different wavelengths. The frequency (f) and wavelength (lambda) of an electromagnetic wave in free space are related by the equation lambda * f = c, so if you know one you can always calculate the other. Electromagnetic radiation with different frequency ranges interacts with matter in different ways. This is why we can see visible light with our eyes, we can see radio waves with antennas, we can see infrared-waves by checking if what they are hitting is heating up, and we can see ultraviolet by using fluorescent materials that absorb electromagnetic radiation in the ultra-violet part of the spectrum, and then re-emit electromagnetic radiation in the visible part of the spectrum where our eyes are most sensitive.
It all moves at the same speed. V light= c = about 3x10^8 m/s.
