What UV light and chemistry are involved in cyanotype printing?

The process is reduction of iron Fe(III) to Fe(II), which then forms ferric ferrocyanide (Prussian blue). It is accomplished by UV rays.

The UV is the fun part: you can safely work the solution under most indoor lightning (which has no UV) and then expose under sunlight (which has UV). The process works in the near visible UV range = 350 - 400 nm, so you can expose your negative under glass.

The process requires light of similar wavelength as acrylic UV gels used in the art nails industry, and some practitioners use a manicure UV curing lamp instead of more traditional sunshine.

The only negative of the process is that requires a lot of UV light to work, and so it is practical only for 1:1 copying (negative and positive of the same size). This is can be overcome by printing the negative digitally on a transparent foil of the required size.

Analogue purists either shoot their negatives on (ultra) large format sheet film (8×10 and bigger) or enlarge their negatives in darkroom. Both requires serious dedication and is best left to advanced users.

This ease of work, together with simple fixing - just a rinse with water to wash away leftover Fe(III) salts - and relatively harmless materials used (compared to other photo processes) makes Cyanotype process great for beginners and even older children.