The luminescence of common light sources (such as lights, flames, and the sun) is due to the fact that when the material is subjected to external energy (such as light energy, electric energy, heat energy, etc.), the electrons in the atom will absorb external energy and transition from the low energy level to the High energy levels, ie atoms are excited. The process of stimulating is an "stimulated absorption" process. The lifetime of electrons at high energy levels is very short (usually 8 to 10 seconds), and they spontaneously transition to low energy levels without external influences. Light (electromagnetic) radiation occurs when the electrons transition.
This radiation is called spontaneous radiation. The spontaneous emission process of atoms is completely a random process. The luminescence of each luminescent atom is independent and not related to each other. That is to say, the radiated light is randomly emitted in all directions in all directions, and the other phases are not polarized. Are not the same. Since the excitation level has a width, the frequency of the emitted light is also not monotonous, but there is a range.
Under normal thermal equilibrium conditions, the atomic number density at a high energy level is far lower than the atomic number density at a low energy level, because at the level of the number density of energy levels, the index decreases with the increase of the energy level E. This is the well-known law of Boltzmann distribution. Obviously, at 20°C, all the hydrogen atoms are almost in the ground state. To make the atoms emit light, the external energy must be provided to make the atoms reach the excited state. Therefore, ordinary generalized light emission includes the two processes of stimulated absorption and spontaneous emission. In general, the energy of the light emitted by such a light source is not strong, and the fact that it is emitted in all directions further disperses the energy.