The radii of the first Bohr orbit of $H$ $(r_{H})$,$He^{+}$ $(r_{He^{+}})$ and $Li^{2+}$ $(r_{Li^{2+}})$ are in the order

Explain: Photoelectric effect.

The shortest wavelength of $H$ atom in the Lyman series is $\lambda_{1}$. The longest wavelength in the Balmer series of $He^{+}$ is:

$A$ quantum will have more energy if :

The radius of the second orbit of a hydrogen atom is the same as that of orbit $n$ of an ion $x$. $n$ and $x$ are respectively:

The photoelectric behaviour of $K$,$Li$,$Mg$,and $Ag$ metals is shown in the plot below. If light of wavelength $400 \ nm$ is incident on each of these metals,which of them will emit photoelectrons?
[Planck's constant $h=6.626 \times 10^{-34} \ J \ s$; velocity of light $c=3 \times 10^8 \ m \ s^{-1}$; $1 \ eV = 1.6 \times 10^{-19} \ J$]