Why cannot the radiation of wavelength 605 nm be used to produce fluorescence radiations?

The correct answer is that the energy of the absorbed radiation must be larger than the emitted radiation for fluorescence to occur. This principle stems from the nature of electronic transitions in molecules. When a molecule absorbs light (radiation), the energy from that light excites electrons to a higher energy state. Subsequently, as these excited electrons return to a lower energy state, they emit light or fluorescence.

In the case where radiation of wavelength 605 nm is being considered, it has a longer wavelength compared to the emitted fluorescence radiation (which typically has a shorter wavelength). Longer wavelengths correspond to lower energy according to the equation ( E = \frac{hc}{\lambda} ), where ( E ) is energy, ( h ) is Planck's constant, ( c ) is the speed of light, and ( \lambda ) is the wavelength. If the absorbed radiation has less energy than that of the emitted radiation, it would not be capable of causing the required electronic transitions, thereby preventing fluorescence from occurring.

In essence, the radiation must have sufficient energy (or equivalently, a shorter wavelength) to promote the electrons to a higher state in order for fluorescence to take place. The understanding of energy levels and electron excitations in fluorescence explains why