A convex lens is placed in contact with the plane mirror. A point object at a distance of 20 cm on the axis of this combination has its image coinciding with itself. What is the focal length of the lens?
In the below diagram, a convex lens is placed in contact with a plane mirror. An object is placed at O.
.PNG)
Object distance, OC=20 cm
According to question,
The image I coincides with O, it means the rays refracted first from the lens and then reflected by the plane mirror must be retracing their path. It would happen when rays refracted by the convex lens fall normally on the mirror i.e. the refracted rays form a beam parallel to principal axis of the lens. Therefore the object O must be at the focus of the convex lens.
i.e. Focal length, F=OC=20 cm
Object distance, OC=20 cm
According to question,
The image I coincides with O, it means the rays refracted first from the lens and then reflected by the plane mirror must be retracing their path. It would happen when rays refracted by the convex lens fall normally on the mirror i.e. the refracted rays form a beam parallel to principal axis of the lens. Therefore the object O must be at the focus of the convex lens.
i.e. Focal length, F=OC=20 cm