LESSON 2

IMAGE FORMATION AND VISION

Light from an external object enters the eye. It is refracted by the cornea into the aqueous humour. The aqueous humour then refracts it to the lens. The lens refracts it to the vitreous humour. The vitreous humour finally refracts light and focuses it to the retina making an image on the retina. The photoreceptors in the retina change the light stimulus into a nervous impulse. The impulse travels along the optic nerve to the brain where interpretation of the image is made. The image formed on the retina is smaller to the real object and it is upside down.

Illustration

CONTROL OF LIGHT AMOUNT ENTERING THE EYE

The iris controls the amount of light entering the eye. It is made up of circular and radial muscles. 

When the circular muscles of the iris contract, the size of the pupil is reduced and less light is allowed in. 

Contraction of the radial muscles widens the pupil so allowing more light to enter the eye. 

In light of low intensity, the pupil widens and in bright light, the pupil reduces in size. This is done to protect the retina from damage by bright light and the wide size of the pupil during dim light allows in enough light of low intensity. 

Control of the amount of light rays entering the eye when in dim light:

In dim light, radial muscles contract,

Circular muscles relax,

Pupil widens and more light is admitted into the eye. Dim Light:

Control of amount of light rays entering the eye in bright light:

Circular muscles of the iris contract,

Radial muscles relax,

Pupil becomes smaller and narrower, Less light is admitted into the eye

ACCOMMODATION OF THE EYE

This is the ability of the eye to change the focal length of the lens when viewing distant or nearby objects.

 Accommodation for a nearby object:

When looking at a nearby object, the ciliary muscles in the ciliary body contract, the suspensory ligaments slacken. This makes the lens short and thick.

This increases the ability of the lens to refract light and reduces the focal length of the lens for the nearby object to be seen clearly.

Illustration

Accommodation for a distant object:

When viewing a distant object, the ciliary muscles in the ciliary body relax. This causes tension in the suspensory ligaments. The suspensory ligaments pull the lens apart making the lens thin and long. This makes the lens to refract less and increase the focal length of the lens.

Illustration