4. Consider the normal incidence of a plane wave from medium 1 (6,,1 = 1) into medium 2 (6r,2 = 4) The ratio of the wavelength in medium 1, A , to that in medium 2, 12, equals
(a) 1,/12 = 1/2.
(b) 1 / 2 2 = 2.
(c) 1/ 12 = 1.
(d) None of the above.