Hydrogen (H), deuterium (D), singly ionized helium (He⁺) and doubly ionized lithium (Li²+) all have one electron around the nucleus. Consider n=2 to n=1 transition. The wavelengths of emitted radiations are lambda₁, lambda₂, lambda₃ and lambda₄ respectively. Then approximately
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For hydrogen-like species, wavelength varies inversely as Z².