Which mirror is used by dentists?

What is a prism?

$A$ convex lens of focal length $20 \, cm$ can produce a magnified virtual as well as real image. Is this a correct statement? If yes,where shall the object be placed in each case for obtaining these images?

Through which point of a convex lens does a ray of light pass without undergoing refraction?

An object is placed at a distance of $18 \ cm$ in front of a concave mirror of focal length $12 \ cm$. Where will the image be formed?

The diagram given below shows an object $O$ and its image $I.$ Copy the diagram and draw suitable rays to locate the lens and its focus. Name the type of lens in this case.