The eye (part 2)
Monacia Williams, Contributor

Hello students! I hope you had a fantastic week of work and play. Did you know that even studying can be fantastic? Yes it can be, but only if you strive to make it so. This week we continue our study of the eye as you continue to prepare for your upcoming examinations. We have looked at the structure of the eye and now we need to see how these structures are used together to help us to see.

How do we see?

• Light rays from the object travels in a straight line to the eyes.
• Light passes through the cornea, pupil and lens at the front of the eye.
• Both the cornea and the lens refract the light rays so that they become focused on the retina.
• The light rays stimulate the light-sensitive cells of the retina.
• The lens varies the amount of refraction so as to focus the light rays to form a sharp, inverted image on the retina.
• The light-sensitive cells send impulses to the optic nerve which then sends them to the brain.
• The brain interprets the impulses and reverts the image, allowing us to see the object the right way up.

The lens has the ability to change its shape in order to permit us to focus so that we can see near and far objects. This is known as accommodation.

Distant/Far object

• Ciliary muscles relax.
• Suspensory ligaments tighten/become taut.
• The lens becomes pulled into a thin shape.
• Less light is refracted. The image is focused on the retina.

Near object

Light rays coming from the near object are going away from each other (diverging).

• Ciliary muscles contract.
• Suspensory ligaments become slack.
• The lens becomes more rounded/fatter.
• More light is refracted. The image is focused on the retina.

As individuals age, the ability of the lens to carry out accommodation becomes lessened and the range of distances over which these individuals can see sharp images is reduced. This loss is more noticeable in dim light than in bright light and often necessitates corrective lenses for close work.

Defects of the eye

These represent another favourite for the examiners. Diagrams similar to the ones below are usually given and you may be asked to:

• Identify the defect/defects shown.
• Draw the lens that would be required to correct the defect/defects.

Long-sightedness/Hypermetropia

This occurs in individuals whose eyeballs are too small or whose lenses are too thin.

• Light rays from far objects are focused on the retina but light rays from near objects focus behind the retina.
• Therefore, the image of the near object falls behind the retina.
• This means that near objects cannot be seen clearly.

This condition can be corrected using convex lenses. This type of lens bends light inwards causing it to converge so that it becomes focused on the retina. Because of this, the lenses are known as converging lenses.

Short-sightedness/Myopia

This occurs in individuals whose eyeballs are too large or whose lenses are too fat.

• Light rays from near objects converge before they focus on the retina.
• The image of the object forms before the retina.
• This means that the object cannot be seen clearly and sharply.

The condition can be corrected using concave lenses. This type of lens spreads the light rays to prevent them from converging too early. The image is now focused on the retina. Because of this the lenses are known as diverging lenses.

New technological advances have enabled specialists to use laser surgery to correct some of these eye defects, thus removing the need for corrective lenses. Contact lenses which fit over the cornea can also be used to correct the defects. These are small objects that come into direct contact with the eye and hence require that the user treats them with care. They should:

• Not be left in the eye for prolonged periods - can cause irritation of the conjunctiva.
• Be inserted with clean, sterilised fingers to prevent the transfer of microbes to the lens.
• Be kept in a sterile medium to prevent the growth of microbes.
• Be inserted with care to prevent damage to the eye and to prevent them from falling out and becoming lost.

Other eye defects

Astigmatism

This is caused by the irregular curvature of the lens and/or the cornea. This can be corrected by the wearing of cylindrical lenses.

Cataract

This occurs when the lens becomes opaque - cloudy. Light rays will not be able to pass through, therefore, the affected individual will not be able to see clearly. The condition can be relieved by surgical removal of the lens and the insertion of an artificial lens.

Glaucoma

• This occurs when too much fluid gathers in front of the eye causing the pressure within the aqueous humour to increase.
• The increased pressure can cause damage to the optic nerve.
• Vision becomes poorer. The individual may experience blind spots within the field of vision.
• Loss of sight can be avoided if the condition is detected early.

Note

Remember the revolution work done by two West Indian scientists in the treatment of glaucoma. Do you remember their names? Of course you do, Drs Manley West and Albert Lockhart. If you did forget the names you certainly could not forget the source of the medication - the ganja plant, Cannabis sativa.

See you next week. Have fun as you study and reconnect with old topics!

Monacia Williams teaches at Glenmuir High School. Send questions and comments to kerry-ann.hepburn@gleanerjm.com