Why no backward wavefront is possible?
This is puzzling. A "mirror" is a "wave optics" device, and a reflection is a "backward wavefront".
In a book it is written
" there cannot be backward flow of energy during propagation of a wave..
and the resultant amplitude of all secondary wavelets at any point on a backward wavefront is zero.
The effects of secondary wavelets cancel out at all points except those lying on forward envelope. So a backward wavefront cannot exist.."
I'm not able to understand this thing
In the future, you should start with this, and not the cryptic first post. Furthermore, in this forum, we require that you cite your source. So if you read it off a book, then the proper way to do this is to provide the author and title of the text.
What your book described is ordinary material, having a "regular", normal index of refraction. But if you are still asking on if it is possible to have a material producing a "backward" wave, then yes, it is. There are a certain class of "metamaterial" that produced a negative index of refraction, often also called left-handed material.
Note that these are more "exotic" type of material that are not usually cover in standard, optics text.
Separate names with a comma.