At what angle of incidence of the beam on the plane-parallel plate the direction of its propagation will not change?

Obviously, at any angle of incidence of the beam on the plane-parallel plate, its direction will change, according to the optical law – the angle of incidence is equal to the angle of reflection.

Even with a vertical (perpendicular) incidence at an angle of 90 °, the beam “bumping” into the plate will be reflected back at the same angle.

An exception to the above is the angle of incidence with a single value, at which the direction of propagation of the beam does not change.

This angle is 0 °. In this case, the beam will propagate parallel to the plate, without encountering obstacles in its path.