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focal British & World English

Relating to the centre or most important part

focal plane British & World English

The plane through the focus perpendicular to the axis of a mirror or lens

focal point British & World English

The point at which rays or waves meet after reflection or refraction, or the point from which diverging rays or waves appear to proceed

focal length British & World English

The distance between the centre of a lens or curved mirror and its focus

bifocal British & World English

Denoting a lens having two parts each with a different focal length, one for distant vision and one for near vision

trifocal British & World English

(Of a pair of glasses) having lenses with three parts with different focal lengths