An object placed in front of a mirror generates an image. If light rays from the object falls on the mirror and are then reflected and converge to form an image, the image thus formed is a real image. If the reflected light rays do not converge but have to be extrapolated backwards to form an image, the image is called a virtual image.
Using ray diagrams, it is possible to determine the type of image formed, while using concave and convex mirrors, based on the distance of object from the mirror. To obtain exact information about the size and magnification of image, and the distance of the image from the spherical mirror, we can use the mirror formula.
The Mirror Formula (also referred to as the mirror equation) gives us the relationship between the focal length (f), distance of object form the mirror (u) and the distance of image form the mirror (v). The mirror formula for a concave mirror is given below. The magnification image formed by a spherical mirror is given by height of image divided by height of object.
Sign Convention for Spherical Mirrors (Concave and Convex Mirrors):
- Distances are to be measured from the pole (vertex) of the mirror marked by point V in the figure
- Distances measured along the direction of the incident ray are positive. Distance measured opposite the direction of the incident ray are negative.
- Distances measured above the principal axis are positive. Distances measured below the principal axis are negative.
The above convention is for both concave and convex mirrors. The figure below shows a concave mirror but the same applies for a convex mirror as well.
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