Why does EM light get a 180° phase shift reflecting from denser media?

[ns_phonon]

Can anyone please provide an intuitive explanation of why phase shift of 180 degrees occurs in the Electric Field of a EM wave,when reflected from an optically denser medium and also why the phase remains same on reflection from a denser to rarer medium?

please try to explain on an atomic/sub atomic/molecular level and "not by the classical example two strings of different materials"

I tried searching for it but, everywhere the result is just used.The reason behind it is never specified or if specified, it is either by using "Frensel equations and mathematics or as an analogy to two strings of different materials connected".

Please explain this phase change on a more microscopic level of interaction of electromagnetic field with matter rather than by an analogy of transverse waves in strings.

 

[UltrafastPED]

These are fundamental properties of waves. So the answer is that the microscopic system responds to the wave.

See http://www.acs.psu.edu/drussell/Demos/reflect/reflect.html for a wave-theory explanation (plus animations & questions to think about).

Note that it is not necessary to understand the microscopic system - you can observe wave properties experimentally - and then predict what will happen with confidence.

Are you familiar with the detailed microscopic analysis of electromagnetic waves impinging upon a dielectric?
Here is a complete lecture, from geometric, to waves, to electromagnetic waves ... with all of the details:
http://www.astro.phys.ethz.ch/spf/people/hanschmi/astobs/notes/files/astobs3.pdf