What is focal length?
PhysicsClass 10CBSEAnswered 28 May 2026
Answer
Focal length is the distance between the optical centre of a lens/mirror and its principal focus.
- Symbol: f | Unit: metres (m)
- Convex lens/concave mirror: positive f
- Concave lens/convex mirror: negative f
Relationship with Power: P = 1/f (in Dioptres)
Focal length is the distance from the lens or mirror to its focus point and determines its optical power.
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