Question

For the given arrangement of a plane mirror and a concave mirror, the light ray is found to retrace its path after reflection from the plane mirror. The point ${}^{\prime }{1}^{\prime }$ through which the incident ray is passing must be

• A. lying between the pole and the focus of concave mirror
• B. lying between the centre of curvature and focus of concave mirror
• C. lying at the centre of curvature of concave mirror
• D. lying at the focus of concave mirror

Answer 1

The correct option is D lying at the focus of concave mirror

In order to retrace the path after striking the plane mirror the light ray must be incident normally after reflection from the concave mirror.

For striking the plane mirror normally, the incident ray after reflection from concave mirror must become parallel to its principle axis.
$\Rightarrow$ The ray incident on concave mirror must pass through its focus.

$\therefore$ Point ${}^{\prime }{1}^{\prime }$ is the focal point or focus of the concave mirror.

CONCEPT INVOLVED: In the problems involving retracing of light rays after reflection, always apply principle of "normal incidence" at that surface. Thereafter complete the further ray diagram as per question demand.