Page 2 - Lesson note refraction by convex lens

Such lenses are called diverging lenses.
o A lens, whether it is a convex lens or a concave lens, has two spherical
surfaces which form a part of a sphere. The centers of these spheres
are called centers of curvature of the lens usually represented by the
letter C.
o Since there are two centers of curvature, we may represent them as
C 1 and C 2.
o An imaginary straight line passing through the two centers of curvature
of a lens is called its principal axis as shown in figure 1.
o The central point of a lens is its optical centre. It is usually represented
by the letter O.
o A ray of light through the optical centre of a lens passes without
suffering any deviation.
o The effective diameter of the circular outline of a spherical lens is
called its aperture.
o In figure 1 (a) you can see several rays of light parallel to the principal
axis are falling on a convex lens. These rays, after refraction from the
lens, are converging to a point on the principal axis. This point on the
principal axis is called the principal focus of the lens.
o Letter F is usually used to represent principal focus. A lens has two
principal foci.
o Similarly in figure 1 (b) several rays of light parallel to the principal axis
are falling on a concave lens. These rays, after refraction from the lens,
are appearing to diverge from a point on the principal axis. This point
on the principal axis is called the principal focus of the concave lens.
o The distance of the principal focus from the optical centre of a lens is
called its focal length represented by letter f .

Image Formation by Lenses
o Lenses form images by refraction of light and type of image formation
depends on the position of the object in front of the lens.
o We can place the objects at
1. Infinity
2. Beyond 2F 1
3. At 2F 1
4. Between F 1 and 2F 1
5. At focus F 1
6. Between focus F 1 and optical center O
o Image formation by a convex lens for different positions of the object is
shown below in the table
Position of Position of Relative size of the image Nature of
the object the image the image

Highly diminished, point Real and
Infinity At focus F 2
sized inverted

Beyond 2F 1 Diminished
Between Real and

2