Page 3 - Lesson note refraction by spherical lens

The central point of a lens is its optical centre. It is usually represented by the letter

O. A ray of light through the optical centre of a lens passes without suffering any
deviation.

4. Aperture
The effective diameter of the circular outline of a spherical lens is called its aperture.

5. Principal focus

 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.

 Letter F is usually used to represent principal focus. A lens has two principal

foci.
 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.

6. Focal length
The distance of the principal focus from the optical centre of a lens is called its focal

length which is represented by letter f .

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