Why does the intensity of unpolarized light remain unchanged when it pass through a quarter wave plate? A quarter wave plate produces a phase difference between e- ray and o-ray. But the intensity is changed for plane polarized light.
An ideal quarter wave plate does not absorb any light.
The speed of light through the quarter wave depends on the orientation of the electric field vector relative to the plate. So one orientation is slowed down more than the other and the thickness of the plate is designed so that there is a change of phase of $90^\circ$ for one plane relative to another.
This represents a change of phase equivalent to a quarter of a wavelength.
An ordinary Polaroid actually absorbs light.
The light which emerges has the electric field oscillating in one plane the plane at right angles having been absorbed.