(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?

Explanation

`*` Listing of any two (out of four) rays and stating their path after reflection from a concave mirror.
• Ray diagram
Using these two rays for the ray diagram when the object is in between the pole and the focus of the mirror.
Detailed Answer:
(i) Two light rays whose path of reflection are known are:
(a) The incident ray passes through the center of curvature: In this case, light after reflecting from the concave mirror moves back in the same path. This happens because light is incident perpendicular on the mirror surface.

(b) 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 and making equal angles with the principal axis.

Let an object is placed between the focus and pole of the concave mirror. Then using above two rays, image of the candle can be located as shown below:

The image is formed behind the mirror. The image is virtual erect and magnified.