Initially invisible ink is invisible because it does not reflect light. It’s transparent. But that can change depending on the type of ink used. The most popular inks used by spies around the world work like this:
Light-activated ink: Under normal light, it’s invisible. But any good spy knows that when ultraviolet light (black light) is shined on it, it glows.
Heat-activated ink: When the invisible ink is exposed to heat, such as under an iron or over a candle flame or 100-watt light bulb, the chemicals in the liquid turn color and the message becomes visible.
Chemically-activated ink: The message is invisible until some chemical is rubbed on it. When this occurs, a chemical reaction makes the message visible.