I don't know if there's a "default" unit, but most people I interact with would use SI units (i.e. km, m, cm, mm, micron, nm, pm). Maybe more to your point, 630

I don't know if there's a "default" unit, but most people I interact with would use SI units (i.e. km, m, cm, mm, micron, nm, pm). Maybe more to your point, 630 nm is the same number of characters and a slightly more familiar unit. Writing a wavelength as 6300 angstroms is a bit like saying a marathon is 421,950 cm. Anecdotally I've only really heard angstroms used in material science / condensed matter physics, where most small structures are small integer numbers of angstroms across.