Cold cathodes may also emit electrons by photoelectric emission. Colder objects emit less radiation, with longer wavelengths. Atoms emit and absorb light at characteristic energies. All objects constantly emit and absorb radiant energy. Therefore, they emit coherent radiation. Neutral atoms or condensed matter cannot emit EUV radiation. Some substances emit light slowly after excitation by more energetic radiation. Most fluorescent lamps use electrodes that emit electrons into the tube by heat. These charged leptons can, if they are energetic enough, emit Cherenkov radiation. The plate, not being heated, does not emit electrons; but is able to absorb them. Most importantly, they do not emit climate-disturbing carbon dioxide. Radioactivity is the ability of a substance to emit high energy particles. The buses are quiet and do not emit pollutants. LED lamps can emit saturated colored light. This is different from radiopharmaceuticals used in nuclear medicine which emit radiation.