question_answer

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?

Answer:

It is a concave lens.

Power of a lens is given by the relation

\[P=\frac{1}{f(in\text{ }metre)}\]

P = 10 D

\[f=\frac{1}{P}=\frac{1}{10}=0.1m\]

Or \[10\text{ }cm\], as lens is concave lens.