Calculate the number of possible spectral lines which may be emitted in Brackett series in $\mathrm{H}$-atom, if electrons present in $9^{\mathrm{th}}$ excited level returns to ground level.

What will be the associated wavelength, if a $0.66\mathrm{kg}$ ball is moving with a speed of $100\mathrm{m}/\mathrm{s}$? $\left(\mathrm{h}=6.6\times {10}^{-34}\mathrm{Js}\right)$

What possibly can be the ratio of the de Broglie wavelengths for two electrons each having zero initial energy and accelerated through $50$ volts and $200$ volts ?