A linearly polarized electromagnetic wave given as E=E0i^coskz−ωt is incident normally on a perfectly reflacting wall z=a. Assuming that the material of the optically inactive, the reflected wave will be given as

When a wave is reflected from a denser medium (or) perfectly reflecting wll made with optically inactive material then the type of wave doesn’t change but only its phase changes by 1800 or π radianGiven here the incident electromagnetic wave is  E¯=E0i^coskz−ωtThis wave is incident normally on a perfectly reflecting infinite wall at z=a . Here the reflected wave in the direction of travelling of wave will be reversed and additional phase of π in the incident waveEr¯=E0i^cosk−z−ωt+π Er¯=−E0i^cos−kz+ωt+π Er¯=E0i^cos−kz+ωtEr¯=E0i^coskz+ωt