If the first emission in the Lyman series of a hydrogen atom occurs at $121.5 \ nm$,what will be the energy difference between the first and second orbits in $kJ/mol$?

The ratio of velocity of electron in the second orbit of $He^{+}$ to the third orbit of $B^{4+}$ is $:-$

$A$ source of monochromatic radiation of wavelength $400 \, nm$ provides $1000 \, J$ of energy in $10 \, seconds$. When this radiation falls on the surface of sodium,$x \times 10^{20}$ electrons are ejected per second. Assume that wavelength $400 \, nm$ is sufficient for ejection of electrons from the surface of sodium metal. The value of $x$ is $......$. (Nearest integer) $(h = 6.626 \times 10^{-34} \, Js)$

Which of the following statements is incorrect?

The ionization energy of a hydrogen atom is $13.6 \, eV$. What is the energy required (in $eV$) to excite the electron from the ground state to the first excited state?

The frequency of the first line of the Paschen series in the spectrum of the $Be^{+3}$ ion is: