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

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 beam travelling along the $X$-axis with a planar wavefront is incident on a medium of thickness $t$. In the region where light is falling,the refractive index varies such that $(dn/dy) > 0$. The light beam on the other side of the medium will emerge:

The idea of secondary wavelets for the propagation of a wave was first given by

$A$ light wave is propagating with plane wave fronts of the type $x+y+z=$ constant. The angle made by the direction of wave propagation with the $x$-axis is

Write the limitation of Huygen's principle.