Before we consider how eyes work or vision happens, we need to understand the structure of our eyes.
Each eye consists of a transparent structure in the front that helps to focus the incoming light. This transparent structure is known as cornea. Behind the cornea lies a ring-shaped colored membrane known as iris. The space between cornea and iris is filled by a clear fluid called aqueous humour. Iris has an adjustable circular opening called pupil. Pupil can expand or contract to regulate the amount of light entering the eye.
A colorless, transparent structure is situated behind the pupil known as the crystalline lens which is held in place by ciliary muscles surrounding it. The ciliary muscles play a significant role in making us see objects. When the muscles relax, they pull the lens and flatten it to enable us to view distant objects and the muscles contract to thicken the lens and see nearby objects.
There are three layers of tissues that make up the eyes –
- The innermost layer is called retina. Retina consists of two types of photoreceptor cells – rods and cones. Rods are found in the peripheral part of retina and provide side vision as well as allowing eyes to detect motion and see in dim light as well as night. Cones are concentrated in the center of the retina, in an area called macula. Cones help us to see in bright light, detect colors and focus on finer details.
- The middle layer is called choroid which contains blood vessels that supply nutrients and oxygen to retina and remove the waste products.
- The outermost layer is called sclera. Sclera gives white color to the eyeball.
When light rays are reflected from an object and enter our eyes through cornea, it bends or refracts the light rays on iris from where it passes through the circular opening called pupil. After passing through pupil, light rays falls on the crystalline lens which changes shape owing to contraction or expansion of ciliary muscles. From lens, light travels through vitreous humour, which is a jelly-like tissue found in the interior chamber of the eyeball, and then strike the retina. The photo-receptor cells, rods and cones, in the retina convert light into electrical impulses. The electrical impulses are transmitted by optic nerve to the brain where an image of the object is produced.
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