Principal Focal Lengths of a Lens – Rankers Physics
Topic: Ray Optics
Subtopic: Lens Makers Formula/ Len's Formula

Principal Focal Lengths of a Lens

Assertion (A): A lens has two principal focal lengths which may be different in magnitude.
Reason (R): The distance of both principal focus from optical centre of lens depend on the two radii of curvature of the lens. Distance of both principal focus from optical centre a lens are same only if radii of curvature of both sides of lens are same.
 
Both (A) & (R) are true and the (R) is the correct explanation of the (A)
Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
(A) is true but (R) is false
Both (A) and (R) are false

Solution:

Assertion (A) is true if the lens is in different media on either side. Reason (R) is false because for a thin lens in the same medium, magnitudes of focal lengths are always equal, irrespective of the equality of radii of curvature.

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