You can see invisible ink with glasses by wearing lenses that block a certain wavelength of light. These lenses can be used by magicians or poker players to reveal hidden markings on the backs playing cards. They can also be used with a camera that has an infrared filter to see invisible markings on paper. There are invisible pens which can be used to write on the backs of papers or other surfaces and will become visible if held up to a source of light.
Over the years, many types of ink invisible have been used to spy and sabotage. During WWII, both the Allies and the Axis powers were constantly working on improving their invisible inks to give them an edge against their enemies. Their goal was to develop an ink which could be easily developed, was odorless and could not have its color revealed by heat or iodine.
Heat activated inks were widely used at this time. This type of invisible ink consisted of any organic liquid that when heated would turn a color and reveal the message. This type of invisible ink was commonly made from lemon juice, apple, onion, 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.
Today, you can still use invisible ink to communicate important messages or to play a game of “hide and seek” with your friends. You can purchase invisible pens from online retailers, and you can also find lenses that will allow you to see the marks on a piece of paper or other surface using your smartphone. For example, the iPhone 8 Plus and newer models have a feature called True Tone that can be enabled to make invisible ink visible on your screen.