Question

In the process called fluorescence, some substances absorb photons of one kind of radiation (e.g) ultraviolet radiation), and give out photons of a different kind (e.g visible light). When infrared radiation falls on these materials, visible light is not emitted. Why not?

If the energy to be given out must be less than that of the incident photons, then the wavelength of the light being readmitted will not be in the visible range. Think about that.