In this article we will discuss about the structure of the eye, with the help of suitable diagrams.
The eye is one of the most important of the receptors. It provides us with information on dimensions, colours and the distance of objects in our environment.
How the Eye Produces a Focused Image:
1. Light rays from an object enter the transparent cornea.
2. The cornea ‘bends’ (refracts) the light rays in towards one another.
3. The light rays pass through the aqueous humour and pupil.
4. The transparent, elastic lens is altered in shape.
It is made:
i. Fatter, to decrease its focal length, or
ii. Thinner, to increase its focal length.
This is called accommodation.
5. The relatively small amount of refraction now produced by the lens brings the rays to focus on the retina.
6. The retina contains light-sensitive cells:
(i) RODS which work well when light intensity is low, and
(ii) CONES which detect colour.
These cells are stimulated by the light of the image, and convert the light energy into electrical energy.
7. Electrical energy, in the form of an impulse, travels along the optic nerve to the brain.
8. The brain de-codes the impulse to produce the sensation of sight.
Other Important Facts:
(i) The image of objects that we are looking directly at (i.e. which are in the centre of our field of vision) falls on a very sensitive part of the retina called the fovea, or yellow spot. This region has far more cones than rods. Cones provide a picture with greater detail and in better colour.
(ii) There are no rods or cones at the point where the retina is joined to the optic nerve. Images formed on this part of the retina are not converted into impulses and relayed to the brain. This region is called the blind spot. We have blind spots in both of our eyes, but are not usually aware of them. Each eye records a different part of our field of view and covers the blind spot of the other.
The ability of the lens to change shape and focus on objects at different distances is called accommodation.
This ability depends on:
(i) The elasticity of the lens
(ii) The existence of ciliary muscles which are used to alter the shape of the lens
(iii) The suspensory ligaments which transfer the effect of the ciliary muscles to the lens.
The Value of having Two Eyes:
Apart from overcoming the effect of the blind spot, two eyes view the same picture from two slightly different positions. This provides vision in three dimensions, the ability to judge distance (and therefore speed), and offers animals a chance of survival even if one eye is damaged.
The ‘Pupil’ (or Iris) Reflex:
Bright light could seriously damage the delicate light-sensitive cells of the retina. The intensity of light falling on the retina is therefore controlled by the iris. It has an antagonistic arrangement of circular and radial muscles.