✔ 最佳答案
If the energy from the incident photons are absorved by the orbital electrons. The electrons jump to higher energy levels. On their falling down to lower energy levls, they may not return to their original level but to level higher than the original level, thus giving out photons of lower frequency than the incident photons.
Hence, when monochromatic photons (photons of single energy) are incident onto a (gaseous) substance, there may be photons of different frequencies re-emitted, these frequencies are those of the charactertic lines of the substance element.
Notice that the amplitude of the EM wave in classical theory, which measures the intensity of wave, corresponds to the NUMBER of photons emitted (not to the frequency of photons).
