Photoelectric emission is observed from a metal surface with incident frequencies $u_{1}u\_1$ and $u_{2}u\_2$ where $u_{1}u\_1$ > $u_{2}u\_2$ If the kinetic energies of the photoelectrons emitted in the two cases are in the ration 2:1, then the threshold frequency $u_{o}u\_o$ of the metal is

The ratio of the energy of a photon of 2000Å wavelength radiation to that of 4000Å radiation is

Atom of an element has Z electrons and its atomic mass is 2Z + 3. The number of neutrons in its nucleus will be 

If an isotope of hydrogen has two neutrons in its atom, its atomic number and mass number will be

The triad of nuclei that is isotonic is

Which of the following is deflected most under the effect of electric field?

The frequency of light having wavelength 500 nm is

The ionization energy of $H{e}^{+}He^+$is $19.6\times {10}^{-18}19.6\; \backslash times\; 10^\{-18\}$ J/atom. The energy of the first stationary state of Li${}^{+2}^\{+2\}$ is

The energy of an electron in the first orbit of hydrogen is -13.6 eV. Which one of the following is the second excited state of electron in hydrogen atom?

The wave number of the radiation emitted when the electron jumps from fourth energy level to second energy level in $H{e}^{+}He^+$is about