What is the radius of the fourth orbit of a hydrogen atom (in $pm$)?

Which electron level would allow the hydrogen atom to absorb a photon but not emit a photon?

$A$ $100 \, W$ bulb emits monochromatic light of wavelength $400 \, nm$. Calculate the number of photons emitted per second by the bulb.

Which formula is used to calculate the energy of a species containing $1$ electron?

The energy of an electron in a hydrogen atom in its ground state is $-13.6 \ eV$. The energy of the level corresponding to the quantum number equal to $5$ is ............. $eV$.

When a certain metal was irradiated with light of frequency $4.0 \times 10^{16} \ s^{-1}$,the photoelectrons emitted had four times the kinetic energy as the kinetic energy of photoelectrons emitted when the same metal was irradiated with light of frequency $2.0 \times 10^{16} \ s^{-1}$. The threshold frequency $(v_0)$ of the metal in $s^{-1}$ is