The electronic energy levels of the hydrogen atom in the Bohr's theory are called

The difference in radii between the fourth and third Bohr orbits of $He^{+}$ (in $m$) is:

The number of maximum possible different spectral lines obtained in the Balmer series,when an electron makes a transition to the ground state from the $5^{th}$ excited state in a sample of hydrogen atoms,is:

Which of the following series of transitions in the spectrum of hydrogen atom falls in the visible region?

When light is directed at the metal surface,the emitted electrons

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: