What is visible light ? Write various view on it. 

Wavefront means

Huygen’s principle of secondary wavelets may be used to

In the adjoining diagram, a wavefront $AB$, moving in air is incident on a plane glass surface $XY$. Its position $CD$ after refraction through a glass slab is shown also along with the normals drawn at $A$ and $D$. The refractive index of glass with respect to air ($\mu = 1$) will be equal to

Huygen's conception of secondary waves