SI units Suppose the caesium standard has the parameters: Velocity: c Energy/frequency: h Time period: Frequency: Wavelength: Photon energy: Photon mass equivalent: Time and frequency The first set of units defined using the caesium standard were those relating to time, with the second being defined in 1967 as "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom" meaning that: 1 second, s, = 9,192,631,770 1 hertz, Hz, = 1/s = 1 becquerel, Bq, = 1 nuclear decay/s = nuclear decays/ This also linked the definitions of the derived units relating to force and energy (see below) and of the ampere, whose definition at the time made reference to the newton, to the caesium standard. Before 1967 the SI units of time and frequency were defined using the tropical year and before 1960 by the length of the mean solar day Length In 1983, the meter was, indirectly, defined in terms of the caesium standard with the formal definition "The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. This implied: 1 metre, m, = = c = 1 radian, rad, = 1 m/m = / = 1 (dimensionless unit of angle) 1 steradian, sr, = 1 m2/m2 = 2/2 = 1 (dimensionless unit of solid angle) Between 1960 and 1983, the metre had been defined by the wavelength of a different transition frequency associated with the krypton 86 atom. This had a much higher frequency and shorter wavelength than the caesium standard, falling inside the visible spectrum. The first definition, used between 1889 and 1960, was by the international prototype meter. Mass, energy, and force Following the 2019 redefinition of the SI base units, electromagnetic radiation, in general, was explicitly defined to have the exact parameters: c = 299,792,458 m/s h = J s The caesium-133 hyperfine transition radiation was explicitly defined to have frequency: = 9,192,631,770 Hz Though the above values for c and were already obviously implicit in the definitions of the metre and second. Together they imply: = = = c = m = h = 9,192,631,770 Hz × J s = J = = = kg Notably, the wavelength has a fairly human-sized value of about 3.26 centimetres and the photon energy