Why doesn't a backward wave exist? (Huygens principle) Since every point on a wavefront act as a source of secondary wave (wavelets) then why do we get only forward wavefront not backward.
Huygens principal says that amplitude of the backward wave is zero, but why and how it happens?
 A: Huygens original description of wave propagation did not adequately explain the backward wave.  Elimination of the backward wave is the reason for the 'obliquity factor' or 'inclination factor' that was added by Fresnel/Kirchhoff. It adjusts the strength of the Huygen's wavelets as a function of the direction of propagation of each segment of the wavelet so that there is no backward wave. It is:
1 + cos θ, where θ is the angle between the normal to the original wavefront and the normal to the secondary wavefront.
Google "obliquity factor" and also see this:
Intuition of inclination factor in Kirchhoff's diffraction law

However the obliquity factor is often considered arbitrary and may not be necessary--see my
"Huygens' Principle geometric derivation and elimination of the wake and backward wave" 
https://www.researchgate.net/publication/340085346
A: Today the principle could be explained as follows.
Every medium has an elasticity and a viscosity. In simple words, the first describes how deeply a body can penetrate or shift the medium over time. The second describes how the medium around is displaced over time.
Imagine a hammer falling lightly on a metal block. The hammer deforms the metal elastically at this point and the metal gives way. In which direction? In all directions. Where the metal is homogeneous, at the same rate. What you get is Huygens (semi)spherical wave.
Note that this wave has a longitudinal and a transverse component. In the direction of the hammer blow, the transverse component predominates (as with sound), and perpendicular to it all around the surface, the longitudinal component predominates.
Absolutely important is that the initial point of the disturbance determines the propagation direction. For a slit, the edges and the wall are the two disturbances. The edges bend the wave behind the edges of the slit, the wall reverse the movement direction.
