According to Einstein, the energy of each packet would be proportional to the light’s frequency, and that suggested an easy way to test the idea: Point a light beam at a metal surface. If the frequency was high enough, at least a few of its energy packets would have enough zing to knock electrons loose from the metal and send them flying out, so that experimenters could detect them. Solar cells work in essentially this way: Light streaming from the sun kicks electrons in the cell up to higher energy levels, producing a flow of electric current.