If light always travels at the speed of light, how does it slow down when passing through air or water? Does it matter if light is made of particles or waves? What’s the difference between phase velocity and group velocity, and how does that all play into this? I discuss these questions and more in today’s Ask a Spaceman! This episode is sponsored by BetterHelp. Give online therapy a try at betterhelp.com/spaceman and get on your way to being your best self. Visit BetterHelp to get 10% off your first month! Support the show: http://www.patreon.com/pmsutter All episodes: http://www.AskASpaceman.com Follow on Twitter: http://www.twitter.com/PaulMattSutter Like on Facebook: http://www.facebook.com/PaulMattSutter Watch on YouTube: http://www.youtube.com/PaulMSutter Read a book: http://www.pmsutter/book Keep those questions about space, science, astronomy, astrophysics, physics, and cosmology coming to #AskASpaceman for COMPLETE KNOWLEDGE OF TIME AND SPACE! Big thanks to my top Patreon supporters this month: Justin G, Chris L, Barbara K, Duncan M, Corey D, Justin Z, Naila, Scott M, Rob H, Justin, Louis M, John W, Alexis, Gilbert M, Joshua, John S, Thomas D, Simon G, Erin J, Jessica K, Valerie H, David B, Frank T, Tim R, Tom Van S, Mark R, Alan B, Craig B, Richard K, Dave L, Stephen M, Maureen R, Stace J, Neil P, COTFM, Stephen S, Ken L, Alberto M, Matt C, Joe R, David P, Ulfert B, Sean M, Edward K, Tracy F, Sarah K, Steven S, Ryan L, Ella F, Richard S, Sam R, Thomas K, James C, Jorg D, R Larche, Syamkumar M, John S, Fred S, Homer V, Mark D, Colin B, Bruce A, Steven M, Brent B, Bill E, Tim Z, Thomas W, Linda C, David W, Aissa F, Marc H, Avery P, Scott M, Thomas H, Farshad A, Matthias S, Kenneth D, Maureen R, Michael W, Scott W, David W, Neuterdude, Cha0sKami, Robert C, Robert B, Gary K, Stephen J, dhr18, Anna V, Matthew G, Paul & Giulia S, Ron D, Steven M, Louis M, Michael C, Alyssa K, Lode D, Roger, Bob C, Patti H, Red B, Benjamin M, BlueDragon, Stephen A, Ian S, James R, Skip M, Robert O, Adam I, Lynn D, Jeffrey C, Allen E, Paul G, Michael S, Jordan, Colin H, Jessica M, Thomas H, and Reinaldo A! Thanks to Cathy Rinella for editing. Hosted by Paul M. Sutter, astrophysicist and the one and only Agent to the Stars (http://www.pmsutter.com).