A linearly polarised electromagnetic wave given as $E=E_0 \hat{i} \cos (k z-\omega t)$ is incident normally on a perfectly reflecting infinite wall at z = a. Assuming that the material of the wall is optically inactive, the reflected wave will be given as:

$E_r=E_0 \hat{i} \cos (k z+\omega t)$

When a wave is reflected from denser medium, then the type of wave does not change but only its phase changes by 180° or π radian.

Thus, for the reflected wave $\hat{z}=-\hat{z}, \hat{i}=-\hat{i}$ and additional phase of π in the incident wave.