$A$ concave mirror can be used to obtain a parallel beam of light from a small lamp.

$A$ convex lens has a focal length of $25\, cm$. Calculate the distance of the object from the lens if the image is to be formed on the opposite side of the lens at a distance of $75\, cm$ from the lens. What will be the nature of the image?

What can be measured using a dioptre meter?

Where is the image formed when an object is placed between the principal focus $(F)$ and the center of curvature $(C)$ of a concave mirror?

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

Where does an object and its image coincide in a concave mirror?