Everything Science Grade 11

13.2 CHAPTER 13. GEOMETRICAL OPTICS

can pass through the lens either from right to leftor left to right, there is afocal point on each side

of the lens (F 1 and F 2 ), at the same distancefrom the optical centrein each direction. (Note:

the plural form of the word focus is foci.)

• The focal length (f) is the distance between the optical centre and the focal point.

Principal axis

F 1 O F 2

Optical centre

f f

(a) converging lens

F 1 O F 2

Optical centre

Principal axis

f f

(b) diverging lens

Figure 13.3: Propertiesof lenses.

Converging Lenses ESBFS

We will only discuss double convex converginglenses as shown in Figure 13.4. Converging lenses are
thinner on the outside and thicker on the inside.

Figure 13.4: A double convex lens is a converging lens.

Figure 13.5 shows a convex lens. Light rays travelling through a convex lens are bent towards the
principal axis. For this reason, convex lenses arecalled converging lenses.