State Huygens' principle. What is the fundamental principle on which it is based?

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

$A$ parallel cylindrical beam of light propagates 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. As the intensity of the light decreases,the radius increases. The initial shape of the wavefront is:

$A$ plane wavefront is incident on a water surface at an angle of incidence $60^{\circ}$. It then gets refracted at an angle of $45^{\circ}$. The ratio of the width of the incident wavefront to that of the refracted wavefront is $\left[\sin \frac{\pi}{4}=\cos \frac{\pi}{4}=\frac{1}{\sqrt{2}}, \sin 60^{\circ}=\frac{\sqrt{3}}{2}, \cos 60^{\circ}=\frac{1}{2}\right]$

How can it be said that light possesses wave nature?

What is visible light? Write various views on it.