Page 58 - Biology_Form_3

Biology for Secondary Schools

Ciliary muscle relaxes

Tight ligament
Object at infinity Image
FOR ONLINE READING ONLY

Thin eye lens
Parallel ray from a
distant object (b)

Figure 2.21: Accommodation of an eye (a) nearby object (b) distant object

Mechanism of vision created and transmitted in the sensory nerve
The eye works on the same principle as the fibres, which join to form the optic nerve.
camera works. Light rays from an object The optic nerve transmits the impulses to
travel through the conjunctiva, cornea, the brain where they will be integrated
aqueous humour, pupil, lens, vitreous to enable a person to see the object in its
humour and finally fall on the fovea correct orientation and its real size.
centralis of the retina. In the process light is
refracted (bent) before it falls on the retina There are two types of light-sensitive cells
to form an image. The curved surface of in the retina which are rods and cones.
both cornea and lens bends the light rays The rods are stimulated by light of low
that enters the eye in such a way that the intensity and are important for vision in
image is formed on the retina. The image dim light. Rods are also responsible for
formed will be upside down (inverted) and black and white vision. In contrast, cones
smaller than the real object as shown in are sensitive to bright light and produce
Figure 2.22. When the light rays strike the mainly a coloured image in the brain.
photoreceptors in the retina, impulses are
Light

Retina
Iris

Inverted image

Object

Lens Optic nerves

Figure 2.22: Structure of mammalian eye showing image formation

51 Student’s Book - Form Three

23/10/2025 11:52:38
BIOLOGY FORM 3 FINAL EDITED 16.09.2025.indd 51
BIOLOGY FORM 3 FINAL EDITED 16.09.2025.indd 51 KAMISHNA 23/10/2025 11:52:38 KAMISHNA