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.
There are materials which absorb photons of shorter wavelength and emit photons of longer wavelength
According to first statement, when the materials which absorb photons of shorter wavelength has high energy of the incident photon on the material and low energy of emitted photon of longer wavelength.
But in second statement, the energy of the incident photon is low for the substances which has to absorb photons of larger wavelength and energy of emitted photon is high to emit light of shorter wavelength. This means in this statement material has to supply the energy for the emission of photons. But this is not possible for a stable substances.