They have appeared in their millions on the streets of Britain and beyond since the 1930s. But what exactly are they, and why do they give coloured light?Matthew Eagles has been fascinated by the yellow glow of SOX streetlights since childhood. In Why SOX Lights? he takes a detailed look from all angles - how they work, the history of their development, their use in art and science as well as streetlighting, how to collect them, research data, their advantages, and more - and explains the cause of their distinctive colour of light.