What is the definition of a wavefront?

The speed of light in water is less than the speed of light in air,confirms the assumption of which theory of light?

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?

The figure shows plane waves refracted from air to water using Huygens's principle. $a, b, c, d, e$ are lengths on the diagram. The refractive index of water with respect to air is the ratio:

$A$ light wave travels in a vacuum along the $x$-axis. Which of the following may represent the wave front?

$A$ parallel beam of light strikes a piece of transparent glass having a cross-section as shown in the figure below. The correct shape of the emergent wavefront will be (figures are schematic and not drawn to scale)-