According to Fermat s principle, light always takes the path which is minimum, maximum or constant in nature.

The velocity of light changes accordingly to its arrival in the same phase. Consider, a plane wave front of light is incident on surface of plane glass tube from air with some angle. Since the speed of light in surface of plane glass tube is less than that in air. As a result the wave front changes its direction towards the normal. If light is incident on a rarer medium from a dense medium the effect will be opposite and light will change its direction away from the normal. So, light does not deviate for normal incidence because each part of wave front approaches medium at the same time and also the extreme nature is to travel without deviation.