The radius of curvature of a spherical mirror is half of the focal length.

$A$ concave mirror and a convex lens are held in water. What changes, if any, do you expect in their focal length?

An object is placed at the following distances from a concave mirror of focal length $15 \, cm$.
$(a)$ $10 \, cm$ $(b)$ $20 \, cm$ $(c)$ $30 \, cm$ $(d)$ $40 \, cm$
Which position of the object will produce:
$(1)$ Virtual image
$(2)$ $A$ diminished real image
$(3)$ An enlarged real image
$(4)$ An image of same size

Which of the following statements is true regarding a real image of an object?

$A$ plane mirror as well as a convex mirror can produce a real image if the object is virtual. How?

$A$ virtual image cannot be focused on a screen. Yet when we see a virtual image,we obviously bring it to the screen that is the retina of our eyes. Is this a contradiction?