question_answer 53)

  There are materials which absorb photons of shorter wavelength and emit photons of longer wavelength. Can there be stable substances which absorb photons of larger wavelength and emit light of shorter wavelength.

Answer:

                  In the first case, the energy of the incident photon on a material is high and the energy of emitted photon is low. In the second case, the energy of the incident photon is low and the energy of emitted photon is high. It means in second case the material has to supply the energy for the emission of photon. This cannot happen for stable substances.