When an electron in an atom falls from higher orbit to a lower orbit a photon gets emitted. Since the electrostatic potential-energy of the electron is negative, because of the attractive force between the proton and the electron, the fall of electron makes its potential-energy more negative. So it is argued here that the energy of the emitted photon is a chunk of positive potential-energy; and since the photon is electrically neutral, it can feel only the gravitational force. Therefore it is proposed here that the photon emitted by an atom might be feeling a repulsive gravitational force; and so it always moves away from the emitting atom. As this photon moves away from the atom, its potential-energy goes on reducing. Then it is shown here that the loss of energy of the cosmologically red-shifted photon is indeed equal to (G me mp/ e^2) times the loss in electrostatic potential-energy of the electron at the same distance D