Invisible ink is a solution or compound that becomes visible upon addition of another substance and can either be made through a chemical or physical process. For example, certain "invisible" solutions only require UV light to become visible, while others require the addition of specific chemicals to set up a reaction.
While soap-based ink is invisible when dried on paper, showing it in front of UV light causes it to show color. When looking at the ink with light of a lower wavelength, such as UV light, the ink glows. On the other hand, when the pH indicator phenolphthalein is used as invisible ink on paper, it is originally colorless. However, on addition of a basic solution such as ammonia, it turns pink and becomes visible.