The kinetic energy of electrons emitted, when radiation of frequency $1.0 \times 10^{15} \mathrm{~Hz}$ hits a metal, is $2 \times 10^{-19} \mathrm{~J}$. What is the threshold frequency of the metal (in Hz )? $\left(h=6.6 \times 10^{-34} \mathrm{Js}\right)$

Identify the pair of species having same energy from the following. (The number given in the bracket corresponds to principal quantum number ( $n$ ) in which the electron is present.)

Which one of the following corresponds to the wavelength of line spectrum of H atom in its Balmer series ? ( $R=$ Rydberg constant)

The radius of third orbit of hydrogen atom is $R \mathrm{pm}$. The radius of second orbit of $\mathrm{He}^{+}$ion (in pm is)