Define principal axis, principal focus and focal length of a convex lens.
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- Principle axis: The principal axis of a lens is a line passing throught the optical centre of the lens and perpendicular to both the faces of the lens.
- Principle focus: The principal focus of a convex lens is a point on its principal axis to which light rays parallel to the principal axis converge after passing through the lens.
- Focal length: The distance of the principle focus from the optical center of a lens is called its focal length.
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