A linearly polarized electromagnetic wave given as 
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
Given
Electromagnetic wave with equation 
Explanation:
When the electromagnetic wave suffers reflection from the perfectly reflecting surface, the direction of the wave is reversed, and the material of the wall is optically inactive so the plane of the wave is also reversed, after reflection i.e.
z=-z
Stoke’s law states, an incident radiation undergoes undergoes a phase change of π after reflection from a denser medium
We know that
And 
1