Wave reflections from mismatched impedances have inverted step waves for DC and inverted phases for AC.  Just like waves in a pool. :)

added:
Do you equate optically denser to higher relative permitivity to lower relative impedance?
Think of the wavelet as a vector which can only reflect a range of inphase or opposite with null in balance of equal density.

"   If terminal impedance is lower the reflection is inverted (-180deg) if higher it is in-phase, if equal, there is no reflection. THis is due to changes in dielectric constant or other physical properties. http://goo.gl/vTwQq

Added:
***This illustration should answer your question intuitively with dark bands caused by out of phase or destructive reflection.***

![refl][1]


  [1]: https://i.sstatic.net/3PvtV.png