What elements orange red spectral line is how the modern day meter is defined?

The orange-red spectral line used to define the modern meter is derived from the element krypton-86. Specifically, it corresponds to the transition between two energy levels in the krypton-86 atom, which emits light at a wavelength of 605.78 nanometers. This definition was established in 1983, where the meter is defined as the distance light travels in a vacuum in 1/299,792,458 seconds. This definition emphasizes the constant speed of light as a fundamental aspect of measurement.