Silver flashes red, yellow, and green.

Tiny grains of silver glow in different colors depending on their size, a new study finds.

Last year, Shuming Nie and his colleagues at Indiana University in Bloomington discovered that bits of metal emit light when excited by a laser, but "in a typical population of particles, at most 1 percent are active," he says. Exploring this phenomenon further, the researchers examined individual silver particles with sensitive light detection techniques to understand how grain size correlates with optical properties.

They report in the Aug. 12 Journal of the American Chemical Society that 70- and 140-nanometer grains emit green and red light, respectively, when excited. Particles that shine yellow either fall in an intermediate size range, around 115 nm, or consist of green and red particles stuck together. If they could sort the particles by size, Nie says, scientists could use them as light emitters in, for example, miniature lasers.