Which will be the first emission line in the infrared region of the hydrogen atomic spectrum?

The energy of an electron in the first Bohr orbit of the $H$ atom is $-13.6 \ eV$. The magnitude of the energy value of an electron in the first excited state of $Be^{3+}$ is $.......... \ eV$ $(nearest \ integer \ value)$.

The ratio of kinetic energies of photoelectrons emitted from a metal by light of frequencies $v_1$ and $v_2$ is $1 : 2$. The threshold frequency of the metal will be:

Arrange the following wavelengths $(\lambda)$ of given emission lines of $H$ atoms in increasing order:
$(a) \ n = 3 \xrightarrow{\lambda_1} n = 1$
$(b) \ n = 12 \xrightarrow{\lambda_3} n = 10$
$(c) \ n = 5 \xrightarrow{\lambda_2} n = 3$
$(d) \ n = 22 \xrightarrow{\lambda_4} n = 20$
Choose the correct option.

Thomson's atomic model is also known as .....

Explain: Photoelectric effect.