$A$ plano-convex lens $(f=20 \,cm)$ is silvered at its plane surface. What will be its new focal length (in $\,cm$)?

$A$ concave mirror is placed on a horizontal table with its axis pointing vertically upwards. Let $O$ be the pole of the mirror and $C$ be its center of curvature. $A$ point object is placed at $C$. It forms a real image at $C$. If the mirror is now filled with water,the image will be formed . . . . . .

$A$ convex lens of focal length $15 \,cm$ is placed in front of a plane mirror at a distance of $25 \,cm$ from the mirror. Where on the optical axis and from the centre of the lens should a small object be placed such that the final image coincides with the object?

$A$ plano-convex lens of focal length $30\,cm$ has its plane surface silvered. An object is placed $40\,cm$ from the lens on the convex side. The distance of the image from the lens is.......$cm$

When a thin convex lens is placed in contact with a concave mirror of the same focal length,the resultant focal length of the system is ......

$A$ lens is placed on a plane mirror. An object needle and its image coincide at $15 \, cm$. The focal length of the lens is......$cm$.