The human eye and its defects

Introduction to the human eye

The human eye is just like a natural camera that God gave us to see the world. It uses a lens to focus light on a screen so that we can see objects clearly. Whether you are looking at a far distance to see a plane or looking closely at a bowl of garri, your eye is constantly working.

Simply put, it is the organ we use to sense light and see everything around us.

Important parts of the eye

Cornea: The clear front part that first bends the light entering the eye.

Iris: The colored part (usually black or brown in Nigerians) that controls how much light enters.

Pupil: The small hole in the middle of the iris where light passes through.

Lens: A transparent, flexible material that changes shape to focus light.

Retina: The 'screen' at the back where the image is formed. It is like the film in an old camera.

Optic Nerve: The cable that carries the image signals to the brain.

Understanding Accommodation

When you look at your phone and then suddenly look at a distant NEPA pole, your eye lens changes its thickness to keep the image sharp. This ability is called accommodation.

It is basically the eye's way of 'autofocusing' so that things don't look blurry.

Eye defects and their corrections

Sometimes, the eye cannot focus properly. This leads to three main problems: Short-sightedness, Long-sightedness, and Presbyopia.

1. Short-sightedness (Myopia): You can see near things clearly but distant things (like the chalkboard from the back of the class) are blurry. The eyeball is too long or the lens is too curved. We fix this using a Diverging (Concave) lens.

2. Long-sightedness (Hypermetropia): You can see far things clearly but reading a book is hard. The eyeball is too short. We fix this using a Converging (Convex) lens.

3. Presbyopia: This happens to older people (like our grandparents) because their eye muscles become weak. They need Bifocal lenses to see both near and far.

Calculation of lens power

The power of a lens used to correct a defect is the reciprocal of the focal length in meters. P = 1 / f. If f is in cm, P = 100 / f. The unit is Dioptres (D).

Example: A student has a far point of 2.0 m. What is the power of the lens needed to see distant objects clearly?

1. The student is short-sighted, so we need a lens that brings an object from infinity to 2.0 m.

2. For a lens to do this, its focal length (f) must be equal to the person's far point, but negative (because it is a concave lens).

3. f = -2.0 m.

4. Power (P) = 1 / f = 1 / (-2.0) = -0.5 D.