What is the definition of a wavefront?

If $AB$ is an incident plane wave front,then what will be the refracted wave front for a lens with refractive index $n_2 > n_1$?

Newton postulated his corpuscular theory on the basis of

Initially,parallel cylindrical wavefronts travel in a medium with a refractive index $\mu(I) = \mu_0 + \mu_2 I$,where $\mu_0$ and $\mu_2$ are positive constants and $I$ is the intensity. The intensity decreases as the radius increases. What happens when it enters the second medium?

$A$ plane wavefront travelling in a straight line in vacuum encounters a medium of refractive index $\mu$. At point $P,$ the shape of the wavefront is:

Huygens' principle is applicable to