What term is used for invisible writing with colorless liquids that can later be made visible by lighting or heating?

What term is used for invisible writing with colorless liquids that can later be made visible by lighting or heating?

• A. Microdots
• B. Transform Domain Techniques
• C. Invisible Ink
• D. Spread Spectrum Techniques

Answer: (C)

Invisible ink is writing done with colorless liquids that stay invisible until activated by heat or light. The trick relies on a reagent that is transparent in normal conditions but changes appearance—often by darkening or fluorescing—when exposed to warmth or ultraviolet light, revealing the hidden message. Classic examples include lemon juice or milk, which darken when heated, and UV-reactive inks that glow under black light. This concept is distinct from microdots (tiny data marks), transform domain techniques (digital processing methods), or spread spectrum techniques (wireless signaling methods).

Invisible ink is writing done with colorless liquids that stay invisible until activated by heat or light. The trick relies on a reagent that is transparent in normal conditions but changes appearance—often by darkening or fluorescing—when exposed to warmth or ultraviolet light, revealing the hidden message. Classic examples include lemon juice or milk, which darken when heated, and UV-reactive inks that glow under black light. This concept is distinct from microdots (tiny data marks), transform domain techniques (digital processing methods), or spread spectrum techniques (wireless signaling methods).