What is meant by dual nature of electrons? Calculate the energy and wavelength of the photon emitted by hydrogen atom when electron makes a transition from $\mathrm{n}=2$ to $\mathrm{n}=1$. Given that the ionisation potential is $13.6 \mathrm{eV}$ ($1\mathrm{eV}=1.6\times {10}^{-19} \mathrm{J}$).