The imaginary line passing through both centres of curvature is called the ______ of the lens. 81
principal axis
The image formed by a ______ lens is always virtual and erect. 84
concave
According to the Cartesian sign convention, the object is always placed on the ______ of the lens. 85
left
The unit of the power of a lens is ______. 85
Dioptre (D)