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Q: The line joining the two focii of a lens is called ................
  • A. paraxial ray
  • B. principal axis
  • C. Focal length
  • D. radius of curvature
Correct Answer: Option B - The straight line passing through the geometrical centre of a lens and joining the two centres of curvature of its surfaces. Sometimes the optical axis of a lens is called its principal axis.
B. The straight line passing through the geometrical centre of a lens and joining the two centres of curvature of its surfaces. Sometimes the optical axis of a lens is called its principal axis.

Explanations:

The straight line passing through the geometrical centre of a lens and joining the two centres of curvature of its surfaces. Sometimes the optical axis of a lens is called its principal axis.