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This is the phenomenon whereby certain chemical elements – such as phosphorus – emit light in the dark. Due to a special configuration of their electrons, they absorb energy from visible light or ultraviolet light. When atoms receive this energy, their electrons are activated and begin to radiate light, even in the dark. After a certain time, no more than a few minutes, the electrons return to their normal state and stop emitting light. “That’s what happens with switches”, says chemist Adalgisa Rodrigues, from the Faculty of Sciences, Philosophy and Letters at USP in Ribeirão Preto. Phosphorus, which gives the phenomenon its name, was discovered in the 17th century, when a German alchemist was trying to synthesize gold from urine.