How invisible, UV light readable only ink works

[Aritra]

Hello everyone,

On asking my teacher, I was told that, UV light readable ink works by changing the frequency of the ultraviolet light to something inside the visible spectrum. While I understood the basic concept, which, after all, was very simple, I couldn't understand how the chemical reduces the frequency of waves. On refraction, the velocity of the light changes and, this is understandable. However, neither refraction, nor reflection change the frequency of light. Also, does this mean that, say, by passing red light through this chemical, infra red light could be produced?

Thanks

[swansont]

The material absorbs the light, and then redistributes the energy through the molecule via transitions that don't radiate visible light — you could be exciting vibrational states of the molecule, for example. A lower-energy transition then gives you a visible photon.

 

It's a little like if you dropped a ball out of a tree. If it dropped straight down it would be like the UV photon, but it first hits a branch or two, giving them energy, and it only then drops to the ground, from a lower height, which is like the visible photon.

 

Yes, you could get IR photons from this process.

[Aritra]

Thanks.