Metre

Metre, basic SI unit of length or distance. It is defined as the distance travelled by light in a vacuum in a time interval of 1/299,792,458 second.

In principle, as the definition implies, the metre can be realized (see Measurement: Units and Basic Standards) by observing the distance that light travels in an accurately timed period in a vacuum (or, strictly, in a region of space where gravitational effects are minimized). However, this encounters practical difficulties. The metre can be realized more conveniently by measuring the absolute frequency of a particular kind of light, relative to the caesium atomic clock, and converting this to a wavelength value, using the relationship frequency × wavelength = speed of light. Here the speed of light, c, has the constant value 299,792,458 m/s (this is true by the definition of the metre). The wavelength of the light is then known precisely, and forms an extremely high-resolution “ruler”. Distances or displacements up to a few tens of metres can be directly measured in terms of this wavelength by means of laser interferometry. This technique is now standard within the precision engineering industries.

Accurate frequency measurements of certain visible laser radiations have been made for this purpose by standards laboratories around the world, and internationally recommended values for these radiations are published. The most commonly used is the 633-nanometre red light from a helium-neon gas laser. This frequency can be electronically servo-controlled to a very stable value, accurate to a few parts in 10¹¹, using a narrow spectral line in molecular iodine vapour.