Given below are some famous numbers associated with electromagnetic radiations in different contexts in physics. State the part of the electromagnetic spectrum to which each belongs.
(a) $2.7 \mathrm{~K}$ [temperature associated with the isotropic radiation filling all space-thought to be a relic of the ‘big-bang’ origin of the universe].
(b) $5890 \AA$ – $5896 \AA$ [double lines of sodium]