Rajatarangini is a historical chronicle of the north - western Indian subcontinent, particularly the kings of Kashmir and was written in Sanskrit by Kashmiri historian Kalhana in the 12th century CE.
Convex mirror is used as rear view mirror in vehicles. Because the image is smaller, more image can fit onto the mirror, so a convex mirror provides for a larger field of view than a plane mirror. This is why they are useful.