The ratio of the area enclosed by the second orbit to the first orbit is ......

The frequency of one of the lines in the Paschen series of a hydrogen atom is $2.340 \times 10^{11} \ Hz$. The quantum number $n_2$ which produces this transition is

The electron present in an excited state of $He^{+}$ ion shows separation (ionization) on collision with an electron having $2.5 \ eV$ energy. What will be the minimum value of $n$ for this electron of $He^{+}$ ion?

Which of the following statements are correct?
$I$) The energy of hydrogen atom in its ground state is $-13.6 \ eV$
$II$) On the basis of Bohr's model,the radius of the $3^{rd}$ orbit of hydrogen atom is $158.7 \ pm$
$III$) The order of radius of the first orbit of $H, He^{+}, Li^{2+}$ and $Be^{3+}$ is $H > He^{+} > Li^{2+} > Be^{3+}$

Which transition in a $H$ atom emits the maximum energy?

Light of frequency $1.6 \times 10^{16} \ Hz$ falling on a metal plate emits electrons that have double the kinetic energy compared to the kinetic energy of electrons emitted when light of frequency $1.0 \times 10^{16} \ Hz$ falls on the same plate. The threshold frequency $(v_0)$ of the metal in $Hz$ is: