Circumference of the first orbit of hydrogen atom is given by the formula :

If in Bohr's model,for a unielectronic atom,the time period of revolution is represented as $T_{n,Z}$ where $n$ represents the shell number and $Z$ represents the atomic number,then the value of $T_{1,2} : T_{2,1}$ will be

The energy of the electron at infinite distance from the nucleus in Bohr’s model is taken as:

The surface of sodium metal is irradiated with radiation of wavelength $x$ nm. The kinetic energy of ejected electrons is $2.8 \times 10^{-20}$ $J$. The work function of sodium is $2.3$ eV. The value of $x$ is . . . . . . $\times 10^2$ nm.

The emission spectrum of hydrogen discovered first and the region of the electromagnetic spectrum in which it belongs, respectively, are

Radius of $1^{st}$ orbit of $H$ and some orbit of $Be^{3+}$ is same. Energy of their orbit of $Be^{3+}$ is ............$eV$.