Thin lenses are divided into:
a) Converging lenses
b) Diverging lenses
Before we study lenses in detail, there are a few important terms that must be deﬁned.
• The principal axis is the line which runs horizontally straight through the optical centre of the lens. It is also sometimes called the optic axis.
• The optical centre (O) of a convex lens is usually the centre point of the lens. The direction of all light rays which pass through the optical centre, remains unchanged.
• The focus or focal point of the lens is the position on the principal axis where all light rays which run parallel to the principal axis through the lens converge (come together) at a point.
(Note: the plural form of the word focus is foci.)
• The focal length (f) is the distance between the optical centre and the focal point.
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