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What is the name of the chemical used in watches to make them glow in the dark?

Radioactive paints are typically used to make watch surfaces glow in the dark. (Radioactive refers to the tendency of an element to break down spontaneously into one or more elements.) These paints do not require activation by an outside light source and will glow for several years. In the past, radium was often used in glow-in-the-dark paints. However, this practice was discontinued when radium was found to give off dangerous gamma rays (a high-energy form of electromagnetic radiation).

The radioactive materials used in watch paints today emit much lower doses of radiation. These levels of radiation are easily blocked by the glass or plastic covering the watch face. The radioactive substances used today include tritium, krypton 85, promethium 147, and thallium 204.

Sources: Brady, George S. Materials...

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