When light is directed at the metal surface,the emitted electrons

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

The radius of the first orbit of $Li^{2+}$ is $X \mathring{A}$. The radius of the third orbit of $He^{+}$ (in $\mathring{A}$) is (in $X$)

The wavelength of the radiation emitted,when in a hydrogen atom an electron falls from infinity to stationary state $n=1$,would be ............... $nm$ (Rydberg constant $= 1.097 \times 10^7 \ m^{-1}$)

What are the $Zeeman$ and $Stark$ effects? What is their significance?

When a metal surface is exposed to a certain frequency of electromagnetic radiation,the kinetic energy of the electron ejected from the metal surface is $0.20 \ eV$. If its work function $(W_0)$ is $4.80 \ eV$,the approximate frequency of the radiation falling on the metal surface in $Hz$ is: