Writing with a pen is invisible, but it can be seen under ultraviolet light. Tell me how to do it easily, not too complicated, preferably at home.

It is not a conventional ink dye, but an organic matter.

This organic matter

The absorption wavelength is not in visible light,

But in the ultraviolet region (which requires that the electronic energy level is not easy to jump)

It is hard to say that an organic compound with a * * * yoke double bond can achieve such ultraviolet absorption. You can go to the organic chemistry laboratory of any university and ask them for some silica gel chromatographic plates. Then, let them separate their organic products, and you can see them.

For colorless organic matter, they took a photo under ultraviolet light to see if the two substances were separated.