The series in the he spectrum that corresponds to the set of transitions where the electron falls from a higher level to the nf = 4 state is called the pickering series, an important series in solar astronomy. calculate the pickering series wavelength associated with the excited state ni = 6.

In this problem, it is necessary to introduce the atomic number Z into the equation. 
E=(2.18×10^-18 J)(Z^2 )|1/(ni^2 )-1/(nf^2 )| E=(2.18×10^-18 J)(2^2 )|1/(6 ^2 )-1/(4 ^2 )|=3.02798×10^-19 J 
Plug the E value calculated into the equation: (wavelength is always positive) 
E=hc/λ3.02798×10^-19 J=hc/λλ=6.56×10^-7 m 
656 nm is in the visible spectrum