Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2016 / S3 / 3 / 5 / a

The Lyman-alpha gap is $3Z^2(\mu /m_e)\mathrm{Ry}/4$, giving $\lambda =4hc/[3Z^2(\mu /m_e)\mathrm{Ry}]$. For heavy lithium nucleus and $Z=3$, $\boxed{\lambda \simeq 13.5\mathrm{nm}}$. A specified lithium isotope would permit the small reduced-mass correction; no isotope is supplied.