An atom with an energy level of 1.8 eV above its ground state emits a photon when returning to that state. The energy of the emitted photon is 1.8 eV, which can be calculated using the formula E = hf, where E is energy, h is Planck's constant, and f is frequency. The discussion highlights the historical contributions of Max Planck and Albert Einstein to the understanding of photon energy and the photoelectric effect, with a focus on the relationship between energy and frequency. The conversation emphasizes the importance of these foundational concepts in quantum mechanics.