How We Can See

The images that we see are made from light that is reflected from objects. After the light reaches the eyes, the brain processes the information to create a vision.

Today, we will learn more about how our eyes make it possible for us to see 😎

Frequently asked on IGCSE Biology: Structure and function of eye 👀

Structure of Eye 

• Cornea: The cornea is the transparent structure at the front of the eye. When light approaches cornea, the cornea refracts light.
• Iris: Iris is the tissue behind the cornea that regulates the amount of light entering the eye by controlling the size of the pupil.
• Pupil: The pupil is the opening at the center of the iris through which light enters the eye.
• Lens: After light is primarily refracted by the cornea, the lens refracts light again to focus light on the retina.
• Retina: The retina is the tissue that lines the back of the eye and contains light receptors, such as rods and cones. These photosensitive cells convert light into electrical impulses.
• Optic nerve: The optic nerve carries impulses between the eye and the brain. The impulse is then processed in the brain to create a vision.
• Vitreous: Jelly-like substance that fills the space of the eye

How Iris Controls How Much Light Enters the Eye

There are two kinds of muscles that make up the iris:

• inner circular muscles
• outer radial muscles

These two muscles of the iris contract and relax to control the size the pupil. The size of the pupil determines how much light enters the eye.

At bright light:

• Circular muscles contract, and radial muscles relax.
• The pupil reduces in size.
• Less light enters the eye to prevent any damage to the eye.

• Radial muscles contract, and circular muscles relax.
• The pupil becomes larger.
• More light enters the eye for a clearer image.

Eye Accommodation

• Eye accommodation is the adjustment of the thickness of the lens to focus the light coming from distant or near objects on the retina.
• Ciliary muscles and suspensory ligaments work together to change the thickness of the lens. Ciliary muscles contract or relax to change the shape of the lens, and suspensory ligaments join ciliary muscles to the lens.

Distant objects:

• Ciliary muscles relax, and suspensory ligaments are stretched.
• There is a high muscle tension on the lens, so the lens becomes thinner.
• Light is refracted only slightly, and light is focused on the retina.

• Ciliary muscles contract, and suspensory ligaments are loosened.
• There is low muscle tension on the lens, so the lens becomes thicker.
• Light is refracted a lot, and light is focused on the retina.

Defects in the Eye that May Affect Vision

• Cataract (cloudy lens) : Since the lens is responsible for refracting light and focusing it on the retina, people suffering from cataract will have decreased vision.
• Detached retina: Retina may be lifted off the wall of the back of the eye. Since the detached retina has no connection to the optic nerve, people with detached retina may suffer from vision loss.
• Myopia (short-sightedness): People with myopia can see close objects clearly but cannot focus properly on distant objects. This is because light is not focused on the retina but rather focused in front of the retina. This happens because either the eyeball is too long or the lens is too thick.
• Hyperopia (long-sightedness): People with hyperopia can see distant objects clearly but cannot focus clearly on close objects. This is because light becomes focused behind the retina. This happens because either the eyeball is too short or the lens is too thin.

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