Images formed by a concave mirror
Fig. 22.4-22.9 are ray diagrams showing the image formed by a concave mirror for different positions of an object placed on the principal axis. In all ‘of these diagrams the object is represented by a vertical arrow OA and the image by lB. Starting with Fig. 22.4 in which the object is placed between F and P, it will be seen that rules 1 and 2 have been used to locate the image of the point A on the object. On drawing the two reflected rays they diverge from a point B behind the mirror. B is therefore a virtual image of A. If the same construction is carried out for a series of other points along the object OA a corresponding set of virtual image points will be formed along a vertical line lB. The image formed in this case is thus larger than the object, virtual, erect and behind the mirror. This type of image is formed by a shaving or make-up mirror and also by the small concave mirror used by dentists for examining teeth.
In Fig. 22.5 the object has been moved to the principal focus. All rays given from any point on the object are reflected parallel to one another. It i~ USt ai, in such cases, to regard the rays as intersecting to form an image at infinity. Fig. 22.6-22.9 are of particular interest, since in these cases real images are forrne.; If a white screen is placed at the image position the image will be formed on it and thus rendered visible from all directions. It is important to be able to distinguish clearly between real and virtual images. A real image is formed b the actual intersection of rays, whereas a virtual image is one forme bYthe apparent intersection of rays when their directions have been produced backwards. The practical distinction is that ‘a real image can be formed on a screen while a virtual image cannot. It must be understood that the use of full and dotted lines in ray diagrams is not a matter of personal preference but is an accepted convention. Full lines are used to represent real rays, objects and real images, while dotted lines are used for virtual rays and images. Also, all rays should be arrowed to show the direction in which the light is travelling. If these rules are followed the diagrams will be made more informative and there will be less likelihood of confusion.
In Fig. 22.6-22.8 the object and image positions are examples of conjugate foci. Conjugate foci are any pair of points such that an object placed at one of them gives rise to a real image at the other. The fact that an object and its real image can be interchanged follows from the principle of reversibility of light. See also page 252.