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Terms Associated with Lenses:
Optical centre
Optical centre is a point at the centre of the lens. it always lies inside the lens and not on the surface. It is denoted by
′
O
′
.
Centre of curvature
It is the centre point of arc of the two spheres from which the given spherical lens (concave or convex) is made. since a lens constitutes two spherical surface, it has two centers of curvature.
Radius of curvature
The distance of the optical centre from either of the centre of curvature is termed as the radius of curvature.
Principal axis
The imaginary straight line joining the two centers of curvature and the optical centre (O) is called the principal axis of the lens.
Focus
The focus (F) is the point on the principal axis of a lens where all incident parallel rays, after refraction from the lens meet or appear to diverge from. For lenses, there are two foci (F
1
and F
2
) depending on the direction of incident rays.
Focal length
The distance between the focus (F
1
and F
2
) and the optical centre (O) is known as the focal length of the lens.
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