Important Definations on Lenses:
Pole
The centre of the spherical refracting surface of the lens is called the pole. The point where the principal axis meets the surface of the lens.
Optical Centre
Optical centre is the point on the principal axis at the centre of the lens.
Centre of Curvature When two spherical surfaces forms a part of a sphere. The centre of these spheres is known as the centre of curvature.
Principal axis
An imaginary line passing through the centres of curvature and the pole is called its principal axis
Aperture
The area of the lens suitable for refraction is called Aperture. The aperture of the lens is the effective diameter of its light-transmitting area.
Focus
Focus is the point onto which collimated light parallel to the axis is focused.
Focal length
The focal length is the distance between the optical centre and the focal point or focus of the lens
Power
The power of the lens is the reciprocal of its focal length. The S.I unit of power is Dioptre.
