There are several ways to see invisible ink. The most important is to find a light source with a specific wavelength. Some people use blacklight or UV bulbs, but others have regular lamps. You can even buy specialized glasses that filter out certain wavelengths to make invisible ink visible.
During World War II spies used invisible pens to send secret messages. The ideal invisible ink should be odorless and reveal itself only after being sprayed by an atomizer. It should also not be detected by iodine or ultraviolet light, nor be affected by heat. The Allies and Axis laboratories competed to create this holy grail invisible ink.
You can create some types of invisible ink using a simple acidic liquid such as lemon juice. You can also use other acids to write hidden messages, including vinegar, apple juice, onion liquid, and even saliva and urine. Some of these organic invisible inks are developed by heating, which makes them appear darker on paper or burn the fibers of the paper they are written on.
Other invisible inks work on the basis of fluorescence. They require a source of light that emits a specific color to be visible. These inks can be used to write secret messages onto cards in card games such as poker. For these inks, a light source that emits blue is usually required to make them visible. This is why most people who use invisible ink with pens wear a pair of sunglasses that can block the specific wavelength of light needed to make the ink appear.