The space between the proton and the electron in a hydrogen atom is:

In the hydrogen spectrum,if $n = 4$,what is the ratio of the number of lines in the emission spectrum in the ultraviolet and visible regions?

The energy of an electron revolving in the $n^{th}$ Bohr's orbit of an atom is given by the expression

Lifetimes of the molecules in the excited states are often measured by using pulsed radiation source of duration nearly in the nanosecond range. If the radiation source has the duration of $2 \, ns$ and the number of photons emitted during the pulse source is $2.5 \times 10^{15}$,calculate the energy of the source.

The ground state energy of $H$ is equal to

The energy required to release $1$ electron from $He^{+}$ is $............$ $eV$.