Visible light passing through a circular hole forms a diffraction disc of radius $0.1 \mathrm{mm}$ on a screen. If X-ray is passed through the same set-up, the radius of the diffraction disc will be

Find the radius and energy of a ${\mathrm{He}}^{+}$ ion  in the states, 

$\left(\mathrm{a}\right) n=2,$

$\left(\mathrm{b}\right) n=3.$

Find the smallest wavelength in emission spectra of

$\left(\mathrm{a}\right) \mathrm{hydrogen},$

$\left(\mathrm{b}\right) {\mathrm{He}}^{+}$

Calculate the angular frequency of revolution of an electron occupying the second Bohr orbit of${\mathrm{He}}^{+} \mathrm{ion}.$

(Use values: $m_e=9.108\times {10}^{-28} \mathrm{g}, h=6.625\times {10}^{-27} \mathrm{erg} {\mathrm{s}}^{-1}\text{ and }e=4.8\times {10}^{-10} \mathrm{esu}$)