<P> Plane mirrors are the only type of mirror for which a real object always produces an image that is virtual, erect and of the same size as the object . Virtual objects produce real images, however . The focal length of a plane mirror is infinity; its optical power is zero . </P> <P> A plane mirror is made using some highly reflecting and polished surface such as a silver or aluminum surface in a process called silvering . After silvering, a thin layer of red lead oxide is applied at the back of the mirror . The reflecting surface reflects most of the light striking it as long as the surface a remains uncontaminated by tarnishing or oxidation . Most modern plane mirrors are designed with a thin piece of plate glass that protects and strengthens the mirror surface and helps prevent tarnishing . Historically, mirrors were simply flat pieces of polished copper, obsidian, brass, or a precious metal . Mirrors made from liquid also exist, as the elements gallium and mercury are both highly reflective in their liquid state . </P> <P> Mathematically, a plane mirror can be considered to be the limit of either a concave or a convex spherical curved mirror as the radius, and therefore the focal length, becomes infinite . </P>

Radius of curvature in case of plane mirror
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