Terms used:

1.Principal axis is a line joining the principal

focus and the optical Centre.

2. Principal focus of a convex lens is a point on

the principal axis to which all rays originally

parallel and close to the principal axis converge

after refraction by the lens.

3. Principal focus of a concave lens: This is

appoint on the principal axis to which all rays

originally parallel and close to the principal axis

appear to diverge after refraction.

4. Focal length: This is the distance between the

principal focus and the optical centre.

5. Optical centre: this is the centre of the lens at

which rays pass un deviated.

CONSTRUCTION OF RAY DIAGRAM

In constructing ray diagram, 2 of the 3 principal

rules are used.

1. A ray parallel to the principal axis is

refracted through the focal point.

2. A ray through the optical centre passes un

deviated i.e. is not refracted.

3. A ray through the principal focus emerge

parallel to the principal axis after refraction.

   Images formed by convex lenses:

The nature of the image formed in a convex lens

depends on the position of the object from the lens.

(a) Object beyond 2f

(b) Object at 2f

C) Object between f and 2f

(c) Object at f