Episode 5 of Neil deGrasse Tyson’s Cosmos series aired last night on Fox. Thanks to Hulu, US viewers can now watch it online. The episode, called “Hiding the Light,” explores the wave theory of light. It moves across 2000+ years of history and eventually gets into the scientific work of Isaac Newton, William Herschel and Joseph von Fraunhofer, before winding up in modern times and touching on big questions contemplated by astronomers. (For a deeper dive into this material, see our collection of Free Astronomy Courses.) If you need to catch up on earlier episodes, you can find them below.