What is focal length?

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.