• question_answer (a) To construct a ray diagram we use two rays which are so chosen that it is easy to know their directions after reflection from the mirror. List two such rays and state the path of these rays after reflection in case of concave mirrors. Use these two rays and draw ray diagram to locate the image of an object placed between pole and focus of a concave mirror. (b) A concave mirror produces three times magnified image on a screen. If the object is placed 20 cm in front of the mirror, how far is the screen from the object?

 (a) The following rays of light are usually used to locate the images formed by a concave mirror: The incident ray passess through the centre of curvature: In this case, light after reflecting from the concave mirror moves back along the same path. Concave mirror The ray incident obliquely to the principal axis: In this case, the incident ray will be reflected back by the reflecting surface of the concave mirror obliquely, making equal angles with the principal axis. When the object is placed between pole and focus of the concave mirror, image is formed behind the mirror which is virtual, erect and magnified. (b) Given:         $m=-3$(As image is real) $u=~-20cm$ $v=?$ We have,          $m=-\frac{v}{u}$ $-3=-\left( \frac{v}{-20} \right)$$\Rightarrow v=-60\,\,cm$ The image is located at a distance of 60 cm in front of the mirror. Thus, the screen is at a distance of 40 cm from the object.