What is the ratio of the radii of the second orbit for $H$,$He^{+}$,and $Li^{2+}$?

In the hydrogen spectrum,if an electron transitions from the $6^{th}$ orbit to the $2^{nd}$ orbit through multiple steps,the number of spectral lines produced is ....

For an emission line of atomic hydrogen from $n_i = 8$ to $n_f = n$,the plot of wave number $(\bar{\nu})$ against $(\frac{1}{n^2})$ will be (The Rydberg constant,$R_H$ is in wave number unit).

$1$ mole of photons, each of frequency $2500 \ s^{-1}$, would have approximately a total energy of:

Which of the following graphs correctly represents the variation of Kinetic Energy $(K.E.)$ of an electron in a Bohr orbit with respect to the principal quantum number $(n)$?

If the threshold wavelength for a metal is $\lambda_0$ and the wavelength of incident light is $\lambda$,what will be the maximum velocity of the emitted electrons?