If the image formed by a lens for all positions of an object placed in front of it is always erect and diminished, what is the nature of this lens? Draw a ray diagram to justify your answer. If the numerical value of the power of this lens is 10 D, what is its focal length in the cartesion system?

The type of the lens is concave lens.

Ray diagram is below

since power of lens is given by relation

Therefore the focal length of lens is - 0.1

m as the lens is concave in nature.