Final answer:
When a plane mirror is used instead of a plane glass plate in Newton's rings arrangement, the interference fringes will disappear.
Step-by-step explanation:
When the plane glass plate in Newton's rings arrangement is replaced by a plane mirror, the interference fringes will disappear. This is because interference fringes are produced by the interference of light waves reflected from the top and bottom surfaces of the glass plate. However, with a plane mirror, there is no longer a gap for the interference to occur, resulting in the disappearance of the fringes.