3.2.3 Discussion of the eigenvalues

1
What is the ground state energy of the electron in an hydrogen atom?

2
How much energy does it take to remove the electron from an hydrogen atom in the ground state?

3
If the electron transitions from an energy level $E_{n1}$ to a lower energy level $E_{n2}$, how may the frequency of the emitted photon of light be computed?

4
What sort of photons do hydrogen atoms emit?