question_answer

(a) An object is placed between two plane mirrors inclined at \[\mathbf{60}{}^\circ \] to each other. How many images do you expect to see? (b) An object is placed between two plane parallel mirrors. Why do the distant images get fainter and fainter?                  (c) Why are mirrors used in search-lights parabolic and not concave spherical? (d) If you were driving a car, what type of mirror would you prefer to use for observing traffic at your back?                                                    

Answer:

                (a) Given\[\theta ={{60}^{\circ }}\]. The number of images, \[n=\frac{360}{\theta }-1=\frac{360}{60}-1=\mathbf{5}\] images (b) The distant images are formed due to multiple reflections. At each reflection, a part of the incident intensity of light is lost due to absorption. So the distant images get fainter. (c) For search light, an intense parallel beam of light is required. This requires a reflector of sufficiently large aperture. When a source is placed at the focus of a spherical concave mirror of large aperture, only the paraxial rays are reflected as a parallel beam. Marginal rays  are reflected as a divergent beam, the intensity of which decreases as per inverse square law. But when a source is placed at the focus of a parabolic mirror, all the rays incident on the mirror are reflected as an intense parallel beam of wide cross-section. Hence a parabolic mirror is preferred in searchlights. (d) Convex mirrors are used in automobiles to see the traffic from behind due to the following two reasons : 1. A convex mirror always forms an erect, virtual and diminished image of an object placed anywhere in front of it. 2. A convex mirror has a wider field of view than a plane mirror of the same size.