Doppler died in 1853, unappreciated for his great discovery. In 1860, Ernst Mach suggested that the Doppler Effect might be observed in spectra of light from stars. Absorption of Waves and Absorptional Redshift. Each material medium, no matter how carefully formed, always contains some irregularities. When a wave in a medium reaches an irregularity, then the wave energy may be absorbed , i.e., swallowed or caught in the medium, in or around the irregularity, i.e., in the absorbing center .
Linear absorption reduces the vibrational energy of particles constituting the wave-carrying medium.
In non-linear absorption , the frequency of the wave decreases. This happens, e.g., when absorbing atoms re-emit light of lower frequency. Even when the number of non-linear absorption events is insignificant compared with the number of linear events, waves received after passing long distances have reduced intensities and frequencies. Sounds from far away have a lower pitch . Radio waves, which traveled over our globe, fade to a lower frequency.