From StarTalk.

In this StarTalk grab bag segment, Neil deGrasse Tyson, joined by co-host Chuck Nice, answers a listener question from optics engineer Yusef Kazemi about photons, light speed, and how light behaves when traveling through materials. Tyson explains that while photons always move at the speed of light between interactions, materials like glass or water slow the overall propagation because photons repeatedly interact with molecules—creating the effect of slower light as it passes through the medium.

From ‘Scientific Martyrs, Life Beyond Our Planet & More! | Cosmic Queries #106": https://youtu.be/5wdEVnuzyqg?si=Fb2RntWE950jlf8g

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