Calculate the radius of the first orbit of $Be^{3+}$. (in $pm$)

The graph that depicts Einstein's photoelectric effect for a monochromatic source of frequency above the threshold frequency is

Which were the major problems before the scientists after the discovery of sub-atomic particles?

$A$ bulb emits electromagnetic radiation of $660 \, nm$ wavelength. The total energy of radiation is $3 \times 10^{-18} \, J$. The number of emitted photons will be :
$(h = 6.6 \times 10^{-34} \, J \cdot s, \, c = 3 \times 10^{8} \, m/s)$

If the velocity of the revolving electron of $He^{+}$ in the first orbit $(n=1)$ is $v$,what is the velocity of the electron in the second orbit $(n=2)$?

In the case of a hydrogen atom,when an electron falls from a higher energy level to the $M$ shell,the corresponding spectral line will form a part of: