The radius of the second excited state of $Li^{2+}$ is ................ $\mathring{A}$.

What transition in $He^{+}$ ion shall have the same wave number as the first line in Balmer series of hydrogen atom?

Which of the following equations represents the velocity $(v)$ of the ejected electrons when a metal is struck with light of frequency $\nu$ and the threshold frequency of the metal is $\nu_0$? ($m_e =$ mass of electron and $h$ is Planck's constant).

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.

Which of the following transitions has the minimum wavelength?

What is the radius ratio for the $2^{nd}$ orbit of $Li^{+2}$ ion to the $3^{rd}$ orbit of $Be^{+3}$ ion?