Mirrors Using Ray Diagrams We can also study the formation of images by spherical mirrors by drawing ray diagrams. Consider an extended object, of finite size, placed in front of a spherical mirror. Each small portion of the extended object acts like a point source. An infinite number of rays originate from each of these points.
To construct the ray diagrams, in order to locate the image of an object, an arbitrarily large number of rays emanating from a point could be considered. However, it is more convenient to consider only two rays, for the sake of clarity of the ray diagram. These rays are so chosen that it is easy to know their directions after reflection from the mirror. The intersection of at least two reflected rays give the position of image of the point object.
Any two of the following rays can be considered for locating the image. (i) A ray parallel to the principal axis , after reflection, will pass through the principal focus in case of a concave mirror or appear to diverge from the principal focus in case of a convex mirror. This is illustrated in Fig. .
(a) and (b). Figure Figure Figure Figure Figure . . .
. . (ii) A ray passing through the principal focus of a concave mirror or a ray which is directed towards the principal focus of a convex mirror, after reflection, will emerge parallel to the principal axis. This is illustrated in Fig.
. (a) and (b). (iii) A ray passing through the centre of curvature of a concave mirror or directed in the direction of the centre of curvature of a convex mirror, after reflection, is reflected back along the same path. This is illustrated in Fig.
. (a) and (b). The light rays come back along the same path because the incident rays fall on the mirror along the normal to the reflecting surface. (iv) A ray incident obliquely to the principal axis , towards a point P (pole of the mirror), on the concave mirror [Fig.
. (a)] or a convex mirror [Fig. . (b)], is reflected obliquely.
The incident and reflected rays follow the laws of reflection at the point of incidence (point P), making equal angles with the principal axis. Figure . Figure . Figure .
Figure . Figure . Remember that in all the above cases the laws of reflection are followed. At the point of incidence, the incident ray is reflected in such a way that the angle of reflection equals the angle of incidence.
Image formation by Concave Mirror Figure . illustrates the ray diagrams for the formation of image by a concave mirror for various positions of the object. Figure . Figure .
Figure . Figure . Figure . Figure .
Figure . Figure . Figure . Figure .
Figure . Figure . Figure . Figure .
Figure . Ray diagrams for the image formation by a concave mirror