Glow in the dark products are luminescent because they contain a chemical known as phosphor. Phosphor absorbs energy in the form of photons and then releases it as light, which is why glow in the dark products must be held by a light source before they will glow.
Phosphor releases the energy slowly, which in turn produces the lasting dim light effect. Fluorescent lights, TV screens and computer monitors are all common products that use phosphor; in each of these cases, light is used to charge the phosphor through different methods. For example, an ultraviolet light charges the phosphor in a fluorescent light, and TV screens utilize thousands of tiny phosphor elements that are energized by an electron beam.