Re: why does 'backward propagation of light'doesn't take place?

Girish,

This is actually a rather complicated question to answer and involves using calculus to prove. If you're interested in the mathematical treatment of this, see Hecht's "Optics" or check out this web page: wave acoustics. But basically, the reason light does not travel backward is because the light travelling forward interacts with the light that would essentially be travelling "backward," causing diffraction. I guess you could say that diffraction stops the backward propagation of light.

This is all due to the wave nature of light. Fresnel was the first physicist to explain this phenomenon, and his theory is now known as the Huygens-Fresnel principle. Later, Kirchoff came along and explained this mathematically, but as I mentioned earlier, it's rather complicated. Follow the link above if you're interested in the finer details.

Hope that helps!

Amber