What is mirror formula ? Under what conditions is it valid ? Obtain an expression for magnification of an image formed by a concave mirror.

What is mirror formula ? Under what conditions is it valid ? Obtain an

1.	What is mirror formula ? Under what conditions is it valid ? Obtain an expression for magnification of an image formed by a concave mirror.
Answer» Solution :Mirror formual is a relation between the object-distance (U), the image-distance (v) and the focal length (f) of the mirror. According to mirror formula : `(1)/(v) + (1)/(u) = (1)/(f)` The mirror formula is true for concave and convex mirrors both, IRRESPECTIVE of the fact WHETHER the image formed is real or virtual. However, proper sign for u, v, f and R be applied as per sign convention followed. CONSIDER the formation of the image A.B. of an object AB by a concave mirror. As shown in Figure the right angled triangles ABP and A.B.P are similar triangles, hence `(A.B.)/(AB) = (PB.)/(PB)` As per sign convention followed, PB = -u, PB. = - v, AB = SIZE of the object = +h and A.B. = size of the image = -h.. Hence, we have `(-h.)/(h) = (-v)/(-u) pr (h.)/(h) = - (v)/(u)` Thus, by definition of magnification of image, we have Magnification, `m = (h.)/(h) = - (v)/(u)`.