In the Bohr model of an atom, the radius of a hydrogen atom in the ground state is $r_1$ and the radius of a $He^{+}$ ion in the ground state is $r_2$. Which of the following is correct?

The work function for a caesium atom is $1.9 \, eV$. Calculate $(a)$ the threshold wavelength and $(b)$ the threshold frequency of the radiation. If the caesium element is irradiated with a wavelength of $500 \, nm$,calculate the kinetic energy and the velocity of the ejected photoelectron.

Which one of the following corresponds to the wavelength of a line in the Balmer series of the $H$ atom spectrum? $(R =$ Rydberg constant$)$

Which of the following transitions emits a photon with the maximum frequency?

If an electron in a hydrogen atom undergoes a transition from the $4^{th}$ stationary orbit and emits a line in the Lyman series,calculate the wavelength of the emitted radiation.

The radii of the first,second,third,and fourth Bohr orbits of hydrogen are respectively: