When an object is placed at $2F$ in front of a convex lens,where will its image be formed?

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

What is the type of the final image,as compared to the object,in the compound microscope?

How will you identify a convex lens by touching it?

The mirror outside the driver's side of a vehicle is usually a convex mirror. Printed on such a mirror is usually the warning "Vehicles in this mirror are closer than they appear". What is the reason for this warning?

The path of a ray of light coming from air passing through a rectangular glass slab traced by four students are shown as $A, B, C$ and $D$ in the figure. Which one of them is correct?