Positronium has $Z=1$ and half the <hydrogen> reduced-mass <energy> scale, so $\boxed{\lambda\simeq243\ \mathrm{nm}}$. The large contrast with lithium is mainly $Z^2$ Coulomb scaling, with an additional factor two from the moving positron's equal <mass>. It is not explained by a different <photon> propagation law.

Solution

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 / b / Solution

 Solution (source code)