# Physics

### Chapter : 4. Light Refraction

#### Some Associated Terms of Lens

Some Associated Terms :
(i) Centre of curvature (C) :
The centre of curvature of the surface of a lens is the centre of the sphere of which it forms a part, because a lens has two surfaces, so it has two centres of curvature. In figure (a) and (b) points, C1 and C2 are the centres of curvature.
The radius of curvature of the surface of a lens is the radius of the sphere of which the surface forms a part. R1 and R2 in the figure (a) and (b) represents radius of curvature.
(iii) Principal axis (C1C2) :
It is the line passing through the two centres of curvature (C1 and C2) of the lens.

Figure : Characteristics of convex and concave lenses
(iv) Optical centre :
If a ray of light is incident on a lens such that after refraction through the lens the emergent ray is parallel to the incident ray, then the point at which the refracted ray intersects, the principal axis is called the optical centre of the lens.

If the radii of curvature of the two surfaces are equal, then the optical centre coincides with the geometric centre of the lens.