question_answer

An infinitely long rectangular strip is placed on principal axis of a concave mirror as shown in figure. One end of the strip coincides with centre of curvature as shown. The height of rectangular strip is very small in comparison to focal length of the mirror. Then the shape of image of strip formed by concave mirror is

A) Rectangle

B) Trapezium        

C) Triangle

D) Square

Correct Answer: C

Solution :

[c] Draw an incident ray along the top side of rectangular strip, which happens to be parallel to the principal axis. After reflection this ray passes through focus. Hence image of all points (for e.g. \[{{O}_{1}},{{O}_{2}},{{O}_{3}}\] ....) on top side of the strip lie on this reflected ray (at \[{{I}_{1}},{{I}_{2}},{{I}_{3}}\].......) in between focus and centre of curvature. Thus the image of this strip is a triangle as shown in figure