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 power of this lens is 10 D, what is its focal length in cartesian system ?

Explanation

If the image formed by a lens for all positions of the object in front of it is always virtual, erect and diminished, the lens is concave.

Power(P)= `- 1/f`
`10=(-1)/(f)`
`f=(-1)/(10)= -0.1 m`
f = 0.1m