system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't we see invisible ink? Why does it only become visible when black light is shone on it?
gpt
There are actually several types of 'invisible ink':

1. Heat-activated
2. Chemically-activated
3. Light-activated

**Heat-activated**
'Inks' like these are based on a heat reaction. Generally speaking, these inks are made from some sort of acidic compound (such as lemon juice!). When heat is applied to these inks, the acidic components react and turn a darker colour -- usually a shade of brown

**Chemically-activated**
These inks are designed using specific two-chemical reactions. Basically, you take a chemical and apply it to some paper or whatever you want to write to. Then when you want to read the message, you apply the secondary chemical, and when those two chemicals come together, they react and turn a different colour. The colour can vary.

**Light-activated**
This is the one you mentioned in your question. These inks work by what is called "fluorescence". What this means is that the chemical used to write the message is a specifically chosen one - one that will 'fluoresce' when an ultraviolet light is applied to it. This is probably the hardest one to explain.
Basically, ultraviolet light is a light whose wavelength is shorter than what the human eye can normally see. The fluorescent chemical absorbs this ultraviolet light, and then re-emits it with a longer wavelength (light with less energy). Because it has a longer wavelength, human eyes can then see it!


It's worth noting that there are a few other types of invisible inks aswell, such as ones that physically modify the medium on which they have been written.