What is the radius of Bohr's fifth orbit for $B^{+4}$ in $\mathring{A}$ (in $.645$)?

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 $R_H = 1.097 \times 10^7 \ m^{-1}$)

Ejection of the photoelectron from metal in the photoelectric effect experiment can be stopped by applying $0.5\, V$ when the radiation of $250\, nm$ is used. The work function of the metal is ................ $eV$

$A$ $50 \, W$ bulb emits monochromatic red light of wavelength $795 \, nm$. The number of photons emitted per second by the bulb is $x \times 10^{20}$. The value of $x$ is $......$.
$[ \text{Given} : h=6.63 \times 10^{-34} \, J \cdot s \text{ and } c=3.0 \times 10^{8} \, m \cdot s^{-1} ]$

The ratio of the radii of the $3^{rd}$ and $5^{th}$ orbits of $He^+$ is ..... . (Atomic number of $He = 2$,$r = 0.529 \times \frac{n^2}{Z} \ \mathring{A}$)

Two electrons are revolving around a nucleus at distances $r$ and $4r$. The ratio of their time periods is