Question

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 Cartesian system?(All India - 2017)

Answer 1

Answer:

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

Given: Power of the lens (P) is -10D (since concave lens hence negative sign is introduced).

Therefore focal length (f)

Hence the focal length is -0.1m or -10cm.