$A$ concave mirror has a focal length $20\,cm$. The distance between the two positions of the object for which the image size is double of the object size is......$cm$

All of the following statements are correct except:

An object is placed at a distance of $40 \,cm$ from a concave mirror of focal length $20 \,cm$. The image formed is . . . . . . .

Suppose that the lower half of the concave mirror's reflecting surface in the figure is covered with an opaque (non-reflective) material. What effect will this have on the image of an object placed in front of the mirror?

In an experiment to find the focal length of a concave mirror,a graph is drawn between the magnitudes of $u$ and $v$. The graph looks like:

$A$ ray of light is incident on a concave mirror. It is parallel to the principal axis and its height from the principal axis is equal to the focal length of the mirror. The ratio of the distance of point $B$ (where the reflected ray intersects the principal axis) to the distance of the focus from the centre of curvature is ($AB$ is the reflected ray).