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

This lens is a Concave lens.

` p = - 10 D " (As lens is concave) " f=?, " " P=(1)/(f)`
So, `f=(1)/(P) rArr (1)/(-10) =-0.1m or -10 cm.`
So, the focal length of the lens is -10 cm.