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CBSE Class 10 Answered

why is the focal length of a convex lens is taken positive and that of a concave lens as negative?
Asked by ca_vipin | 21 Nov, 2010, 12:00: AM
answered-by-expert Expert Answer
Dear student
In a convex lens , since the parallel beam of light travelling parallel to the principal axis actually meets at a point(second focus) , the focus is called real focus and its focal length is taken as positive.
In a concave lens , since the parallel beam of light travelling parallel to the principal axis appear to diverge from this  point(second focus) , the focus is called virtual focus and its focal length is taken as negative.
We hope this clarifies your doubt.
Regards
Team
Topperlearning
Answered by | 21 Nov, 2010, 02:34: PM

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