LEDs, or light-emitting diodes, were first conceived well over a century ago. But it took another half century before they were commercially available, and half a century again before they became ubiquitous. And now they're in everything from our phones to our holiday lights to special effects in film. On a special Butterfly Effect edition, we trace the development of the LED – and ask what might come next. With guests Carrie Meadows, Zheng-Hong Lu and Kevin McGeagh.