CBSE Class 10 Science 2017 · Paper 31/1 · Question 16
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
Concave (diverging) lens; its image is always virtual, erect and diminished; D gives cm.
The lens is a concave (diverging) lens: for every position of the object it forms a virtual, erect and diminished image between its focus and its optical centre, as the ray diagram shows. A concave lens has a negative power, so D and m m cm.
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