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!

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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).