Which one of the following spherical lenses does not exhibit dispersion? The radii of curvature of the surfaces of the lenses are as given in the diagrams.

Which has more luminous efficiency?

An object is placed in front of a convex mirror at a distance of $50\, cm$. $A$ plane mirror is introduced covering the lower half of the convex mirror. If the distance between the object and the plane mirror is $30\, cm$,it is found that there is no parallax between the images formed by the two mirrors. The radius of curvature of the convex mirror will be......$cm$.

$A$ photograph of the moon was taken with a telescope. Later on, it was found that a housefly was sitting on the objective lens of the telescope. In the photograph:

In an experiment for the determination of focal length of a convex mirror,a convex lens of focal length $20 \ cm$ is placed on an optical bench and an object pin is placed at a distance $30 \ cm$ from the lens. When a convex mirror is introduced in between the lens and the real and inverted image of the object,the final image of the object $O$ is formed at $O$ itself. If the distance between the lens and the mirror is $10 \ cm$,then the focal length of the mirror is.......$cm$

In the figure shown,a point object $O$ is placed in air on the principal axis. The radius of curvature of the spherical surface is $60\, cm$. Let $I_f$ be the final image formed after all the refractions and reflections. Which of the following statements is correct?