Wearing glasses that block a specific wavelength of light allows you to see invisible marks. These lenses are used by poker players and magic show performers to reveal hidden marks on the back of playing cards. These lenses can be used with an infrared-filtering camera to see invisible marks on paper. There are even invisible pens you can use to write messages behind papers or on other surfaces. They will become visible once held up against a light source.
Over the years, several types of invisible inks have been used for spying and sabotage. During WWII both the Allies as well as the Axis powers worked on improving their invisible pens to give them a competitive edge over their enemies. Their goal was to create an odorless, easily developed ink that could not be revealed by heat, detected with iodine or exposed to ultraviolet light.
Many of the invisible inks that were used during this time were heat activated. This type of invisible ink consisted of any organic liquid that when heated would turn a color and reveal the message. Common examples of this kind of invisible ink included lemon juice, apple juice, onion juice, vinegar and milk. Spies have been known to use invisible ink to etch messages on skin and then spray the atomizer with them for development.
You can still use invisible ink today to communicate important information or to create a game of hide and seek with your friends. You can buy invisible pens online, and there are also lenses that allow you to see marks on a paper or another surface using your smartphone. True Tone, a feature on the iPhone 8 Plus and other newer models can be enabled in order to make invisible marks visible on your screen.