What makes highlighters neon?

Image by Hebi B. from Pixabay

Question from Ben Reed

Highlighter ink contains fluorescent compounds that absorb ultraviolet (UV) light and emit this energy as visible light, creating extra luminous ‘neon’ colours (although these colours have nothing to do with neon gases). When UV light hits dyes within fluorescent ink, its electrons absorb high-energy UV wavelengths of light, entering an ‘excited’ state. As the electrons drop back to their original state, this excess energy is

emitted as visible light. While a normal yellow ink absorbs and re-emits wavelengths within the visible light spectrum, a fluorescent ink absorbs portions of both visible and UV light, effectively converting the UV light into visible light and emitting up to twice as much visible light as a standard ink. This effect is even more obvious if you shine a UV light onto highlighter ink in a darkened room – the UV light is converted into visible light, making the ink glow

Answered by Alexandra Cheung for Brain Dump in How It Works issue 99.

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