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Question
If the image formed by a lens for all positions of the object placed in front of it is always virtual, erect and diminished, state the type of the lens. Draw a ray diagram in support of your answer. If the numerical value of focal length of such a lens is 20 cm, find its power in new cartesian sign conventions.
Solution
A concave lens will always give a virtual, erect and diminished image, irrespective of the position of the object.
Given, Focal length f = −20 cm = − 0.2 m (negative for convex lens)
\[P = \frac{1}{f(metres)} = \frac{1}{- 0 . 2 m} = - 5 D\]
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