• In vacuum vi = i = 2.99792 x 108 m s-1 ( 3.00 x 108 m s-1 ) = c
• In air vi is essentially = c
• vi smaller in other media because of interactions of the EM field with the electrons in atoms and molecules of the medium.
• Since is constant, when vi decreases, must decrease.
• i decreases when an EM field moves through a medium other that air.