Therefore only when we apply mirror formula 1/u + 1/v = 1/f, when we put f = infinite then we will get -u = v, which is true for all plane mirror. So, technically a plane mirror never going to be a circle. That the image distance is equal to object distance. now radius of this circle is infinity. however in a plane mirror when two rys are parallel to the axis, they will strike the mirror at right angles. therefore focal length = radius/2= infinity/2= infinity [citation needed] The focal length of a plane mirror is infinity; its optical power is zero. Focal length is the point half of radius. If the images formed by the two mirrors coincide, find the focal length of the convex mirror. well,focal length is distance btwen the focus and the mirror,and focus is the point of convergence or divergence.so in case of plane surface there is no conververgence,so the focus does not exist.so how come focal length is infinity? So a small part of length of circumference will behave like a plane mirror. focal length is the length at which parallal rays vould converge. Virtual objects produce real images, however. The focal length of plane mirror is infinite. But, in hypothetically we say it will be a circle of infinite radius. Hence, focal length is also INFINITE. 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. A plane mirror is inserted between the object and the convex mirror at a distance of 40 cm from the object with the reflecting surface of the plane mirror facing the object. A plane mirror does not change the size of the (virtual) image reflected from it. What is the focal length of a plane mirror of size 5cm. Therefore, the magnification is 1. CONSIDER PLANE MIRROR TO BE PART OF A CIRCLE OF INFINITE RADIUS. Sobigger the circle gets the more its radius will be. x5 cm? So in the sameway as the curvature of the sphere gets less and less it … This radius is obtained by extenteding the edges of lens to obtain a imaginary circle. And then we get center of circle, C. Focal lenth is half of radius. Focal length is half of radius of curvature of the mirror. according to the laws of reflection for a plane mirror, the angle of incidence will be equal to the angle of reflection. - 17653281