f, the focal length, is positive for a concave mirror, and negative for a convex mirror. Since, the centre of curvature and focus lies behind the convex mirror, so radius of curvature and focal length are taken as + (positive) in the case of convex mirror. Since, the centre of curvature and focus lies behind the convex mirror, so sign of radius of curvature and focal length are taken as + (positive) in the case of convex mirror. A concave mirror will have a positive focal length. The P’AP and Q’AQ triangles are similar so that we can derive the relationship between the object distance and the image distance with the object height and the image height: This equation is written again as below by adding m: h = the object height (positive if the object is above the principal axis of the convex mirror or the object is upright. community of Class 10. (ii) image distances (v) for a convex mirror. For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci (or focal points) of the lens.For a converging lens (for example a convex lens), the focal length is positive, and is the distance at which a beam of collimated light will be focused to a single spot. positive. According to this statement, the equation of the convex mirror changes to: do = the object distance, di = the image distance, f = the focal length, Always remember the sign rules of the convex mirror when using this equation to solve the problems of the convex mirrors. So the image distance is positive here. Giving reasons, state the signs (positive or negative) which can be given to the following: (i) object distance (u) for a concave mirror. When the image distance is positive, the image is on the same side of the mirror as the object, and it is real and inverted. Since, the centre of curvature and focus lies behind the convex mirror, so radius of curvature and focal length are taken as + (positive) in the case of convex mirror. By continuing, I agree that I am at least 13 years old and have read and Therefore : Based on the sign rules of the convex mirror, this equation can be changed to the equation of the concave mirror, if the image distance (di) is given a negative sign because the beam of light does not pass the image and focal length (f) is also given a negative sign because the focal point of the convex mirror is not passed by light (see the figure of the image formation above). you can easily verify that the focal length is in the positive x direction for a convex lens while it is along the negative x direction for a concave lens. If the image is behind the mirror surface that reflecting light, where light does not pass through the image, then the image distance is negative (virtual image). Conversely, if the object is below the principal axis of the convex mirror, the object height is negative (object is inverted). Parallel rays of light reflected from the mirror seem to originate from the point F at the focal distance \(f\) behind the mirror. If the object is above the principal axis of the convex mirror, the object height (h) is positive (object is upright). Based on the figure below, there are two beams of light to a convex mirror, and the convex mirror reflects the beam of light. Properties of image formed by convex mirror. Thin lens approximation. The focal length and power of a convex mirror are negative, since it is a diverging mirror. is done on EduRev Study Group by Class 10 Students. If the object is above the principal axis of the convex mirror, the object height (h) is positive (object is upright). (i) Object distance (u) for a Concave mirror should be mentioned negative (-ve). If the magnification of image > 1 then the size of the image is greater than the size of the object. Question bank for Class 10. Virtual images will be located behind the mirror surface. image distance. Although the focal length f of a convex mirror is defined to be negative, we take the absolute value to give us a positive value for R. The radius of curvature found here is reasonable for a cornea.