Mainly after writing, ink is a kind of volatile transparent liquid, which can be completely integrated with the carrier after writing, so that our naked eyes can not see it. Specifically, there are many ways to show invisible words. If it is written by milk or lemon juice, it can be displayed by heating; if it is written by colorless indicator, it can be displayed by adjusting acid and alkali; it can also be displayed by oxidation-reduction method, etc. Of course, the invisible pen on the market is a method of ultraviolet color. The principle is relatively simple, because our naked eye can only see the spectrum of visible light. Ultraviolet, shorter than visible light, is invisible to the naked eye. Similarly, infrared, longer than visible light, is invisible to the naked eye. Therefore, it is impossible to recognize the characteristics of UV and IR by using the normal conditions of the naked eye, so UV invisible ink and IR invisible ink are born on the market. At present, the two are mainly used in anti-counterfeiting business
i buy some invisible ink marked card and contact lens，put on a special contact lens. You will see the above mark, which is a special kind of ink that cannot be seen by the naked eye.