What is an emission line?

The following diagram indicates the energy levels of a certain atom. When the system moves from the $4E$ level to the $E$ level,a photon of wavelength ${\lambda _1}$ is emitted. The wavelength of the photon produced during its transition from the $\frac{7}{3}E$ level to the $E$ level is ${\lambda _2}$. The ratio $\frac{{{\lambda _1}}}{{{\lambda _2}}}$ will be:

Some of the wavelengths observed in the emission spectrum of neutral hydrogen gas are $912, 1026, 1216, 3646, 6563 \, \mathring{A}$. If broadband light is passing through neutral hydrogen gas at room temperature,then the wavelength that will not be absorbed strongly is .................. $\mathring{A}$.

The energy of a hydrogen atom in its ground state is $-13.6 \,eV$. The energy of the level corresponding to $n=7$ is ...... $eV$.

Energy levels $A, B$ and $C$ of a certain atom correspond to increasing values of energy, i.e., $E_A < E_B < E_C$. If $\lambda_1, \lambda_2$ and $\lambda_3$ are the wavelengths of radiations corresponding to transitions $C$ to $B, B$ to $A$ and $C$ to $A$ respectively, which of the following relations is correct?

The ground state energy of a hydrogen atom is $-13.6 \, eV$. When its electron is in the first excited state,its excitation energy is ...... $eV$.