The wavefront of a wave has what direction with respect to the wave motion?

What is the significance of Huygens' conception of secondary waves?

$A$ plane wavefront of width $x$ is incident on an air-water interface, and the corresponding refracted wavefront has a width $y$. The refractive index of air with respect to water in terms of distances $w$ and $z$ is:

Explain how to obtain a new wavefront at time $\tau$ using Huygens' principle for a plane wavefront.

Huygens' wave theory allows us to know

Explain how the Corpuscular theory predicts the speed of light in a medium,such as water,to be greater than the speed of light in a vacuum. Is this prediction confirmed by the experimental determination of the speed of light in water? If not,which alternative picture of light is consistent with the experiment?