MCQ practice · SSC Physics Board questions · Chapter 9: Refraction of Light
What is the straight line joining the centres of curvature of a lens called?
Think it through first, then pick an option.
- a focus
- b centre of curvature
- c optical centre
- d principal axis
Answer and explanation
Correct answer: d principal axis
Right. The straight line joining the two centres of curvature (C₁ and C₂) of a lens is the principal axis. This is beyond the book, but a board asked it.
Why the other options are wrong
- a focus: The focus is where parallel rays meet or seem to spread from. The line joining the two centres of curvature is the principal axis.
- b centre of curvature: A centre of curvature is the centre of a sphere, a single point. Joining the two centres of curvature gives a line, the principal axis.
- c optical centre: The optical centre is a point in the lens through which a ray does not bend. The line through the two centres of curvature is the principal axis.
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