Atomic Physics (Question 1 (8 Marks)) — UPSC Physics Optional IFoS 2014 Paper II

An atom in an excited state of 4\cdot 7\text{ eV} emits a photon and ends up in the ground state. The lifetime of the excited state is 10^{-13}\text{ s}. Calculate the energy uncertainty and obtain the spectral line width in wavelength of photon.