According to Snell's law of refraction, the refractive index of a medium is given by, \(\mu=\frac{\sin\left(i\right)}{\sin\left(r\right)}\), where i is incident angle and r is refracted angle. For normal incidence, i = 0, then r = 0 as μ ≠ 0, the minimum value of μ is 1 but never equal to 0. So, a normally incident wavefront does not deviate when it travels from one medium to another.