Orange light emitted from krypton-86 atoms was used to define:

Explanation

The meter is defined in terms of the speed of light in a vacuum (c) and the duration of a specific number of oscillations of a cesium-133 atom. However, before the current definition, the meter was initially defined by a prototype physical object, the International Prototype of the Meter, and later by the wavelength of the orange light emitted by krypton-86 atoms. The current definition based on the speed of light was adopted to provide a more stable and precise standard.