# Lens:

A Lens is a combination of two refracting surfaces out of which at least one is curved

Types of Lenses:

# Some imported terms:-

(i) Principle axis:

It is a straight line passing through the centre of curvatures of two surfaces of a lens                

(ii) Optical centre (o): It is the central point of the lens.

(iii) Principle Focus (F): It is the point on the principle axis at which all the rays II to principal axis meet after passing through the lens.

(iv) Focal length (f): It is the distance between F and O.

(v) Aperture (LL’): It is the effective width of a lens from which refraction takes place                         

# Rules for the formation of image by a lens.

(i) A ray of light incident II to the principal axis of a lens passes through the focus after refraction.

(ii) A ray of light passing through O suffers no deviation.

(iii) A ray of light coming from an object through the focus of a lens becomes II to the principal axis.