Optical centre:

Optical centre is the centre of the lens when light passes through the optical centre after refraction, refracted ray becomes parallel to the incident ray.







Principal axis:

The axis which connect the two centre of curvature of the two surfaces of the lens is called principal axis.



 

Focus:

In case of double convex lens, the rays which are parallel to the principal axis after refraction they converges to a point which lies on the principal axis is called Focus. as shown below





 

On the other hand in case of a concave lens, the rays which are parallel to the principal axis after refraction the rays appears to be diverge from a point which lies on the principal axis. That point is called focus. as shown below








Light from focus:

In case of convex lens the rays which are passing through the focus, after refraction they became parallel to the principal axis. as shown below

 


In case of concave lens the rays which are appearing to meet at focus, after refraction they became parallel to the principal axis. as shown below