Physics Technology Update (4th Edition)

Published by Pearson
ISBN 10: 0-32190-308-0
ISBN 13: 978-0-32190-308-2

Chapter 29 - Relativity - Conceptual Questions - Page 1040: 2

Answer

Because the light travels through a medium.

Work Step by Step

Since the light travels through a medium (glass), its speed can change. The speed of light is only guaranteed to be $c$ in a vacuum.
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