This lecture discusses the emission of a photon by an excited atom, determining its frequency and the uncertainty in its energy. By applying conservation of energy, the wavelength of the photon is found to be 600 nanometers, with a frequency of approximately 4.3 x 10^14 Hz. The uncertainty in the photon's energy is calculated to be around 3.3 micro electron volts, indicating a small distribution around the energy value of 1.8 electron volts.