What is the ratio of the radius of the second Bohr orbit to the first Bohr orbit of the $H$ atom?

What is the value of the frequency of radiation when a transition occurs between two stationary states that differ in energy by $\Delta E$?

For ionising an excited hydrogen atom,the required energy in $eV$ will be -

Calculate the wave number of a photon emitted during the transition from the orbit $n = 2$ to $n = 1$ in a hydrogen atom $(R_{H} = 109677 \ cm^{-1})$. (in $cm^{-1}$)

$A$ bulb of $40 \ W$ is producing light of wavelength $620 \ nm$ with $80 \%$ efficiency. The number of photons emitted by the bulb in $20 \ s$ is $(1 \ eV = 1.6 \times 10^{-19} \ J, hc = 12400 \ eV \ \mathring{A})$

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