In the equation y = A sin (kx-wt), 'y' represents the displacement of particles of the medium about the mean position.

The speed of light is the universal speed limit, and also a universal constant. But the measured speed of light may be varied according to the change in optical density.

The speed of light depends on:

1. Optical Density
2. Permittivity of the medium
3. Permeability of the medium

If the source is linear, we can get cylindrical wavefronts.

We can obtain a spherical wavefront from the point source.

If a point source of light is kept at the focus of a convex lens, we can get parallel waves emerging out from the lens, which form a plane wavefront.

The speed of light is considered the universal speed limit. Yet, the measured speed of light varies with the change in optical density of the medium. Light propagates faster in the rarer medium and slower in the denser medium.

In other words, it also varies along with the change in permittivity and permeability of the medium, as the speed of light is given as the reciprocal of the square root of the product of permeability and permittivity of the medium.

Wavefront is the locus of the adjacent points vibrating in the same phase. Hence, the wavefront also travels in the direction of propagation of energy by the wave. And, we know that the flow of energy does not happen from the lower potential to the higher potential. Therefore,there is no backward wavefront, flowing from lower disturbance to higher disturbance.

The shape of a wavefront is determined by two factors:

a) Shape of source
b) Distance from source

A point source produces spherical wavefronts.
Similarly, a linear source will produce cylindrical wavefronts.
If the source is at an infinite distance, the observer will observe plane wavefronts.

The direction of propagation of rays is perpendicular to the wavefront.

Yes, Huygen's principle is also applicable to sound waves.

The particle theory of light explains Einstein's Photoelectric effect.