"The reflection from a denser medium causes an additional phase change of pi or path change of lamda/2 while reflection from rarer medium doesn't cause any phase change."

can anyone explane me how this happens?

"The reflection from a denser medium causes an additional phase change of pi or path change of lamda/2 while reflection from rarer medium doesn't cause any phase change."

can anyone explane me how this happens?

 

It's a result of the boundary conditions. Energy has to be conserved, of course, and that depends on the square of the amplitude, but you also have the (for light) continuity of the field across a boundary. If you solve that, you find that the reflected wave has to have the opposite direction, i.e. be pi out of phase, for waves going into a higher index (optical density).

 

 

For a rope, or equivalent, IIRC the boundary condition is that slope must be the same (i.e. no kinks)

here's an applet that shows the effect, with a few equations but not the full derivation

http://www.surendranath.org/Applets/Waves/TwaveRefTran/TwaveRefTranApplet.html