How the Human Eye Works?

The human eye has been called the most complex organ in our body. It's amazing that something so small can have so many working parts.

Parts of the Eye

Aqueous Humour
The aqueous humour is a jelly-like substance located in the anterior chamber of the eye.

Choroid
The choroid layer is located behind the retina and absorbs unused radiation.

Ciliary Muscle
The ciliary muscle is a ring-shaped muscle attached to the iris. It is important because contraction and relaxation of the ciliary muscle controls the shape of the lens.

Cornea
The cornea is a strong clear bulge located at the front of the eye (where it replaces the sclera - that forms the outside surface of the rest of the eye). The front surface of the adult cornea has a radius of approximately 8mm. The cornea contributes to the image-forming process by refracting light entering the eye.

Fovea
The fovea is a small depression (approx. 1.5 mm in diameter) in the retina. This is the part of the retina in which high-resolution vision of fine detail is possible.

Hyaloid
The hyaloid diaphragm divides the aqueous humour from the vitreous humour.

Iris
The iris is a diaphragm of variable size whose function is to adjust the size of the pupil to regulate the amount of light admitted into the eye. The iris is the coloured part of the eye (illustrated in blue above but in nature may be any of many shades of blue, green, brown, hazel, or grey).

Lens
The lens of the eye is a flexible unit that consists of layers of tissue enclosed in a tough capsule. It is suspended from the ciliary muscles by the zonule fibers.

Optic Nerve
The optic nerve is the second cranial nerve and is responsible for vision. Each nerve contains approx. one million fibres transmitting information from the rod and cone cells of the retina.

Papilla
The papilla is also known as the "blind spot" and is located at the position from which the optic nerve leaves the retina.

Pupil
The pupil is the aperture through which light - and hence the images we "see" and "perceive" - enters the eye. This is formed by the iris. As the size of the iris increases (or decreases) the size of the pupil decreases (or increases) correspondingly.

Retina
The retina may be described as the "screen" on which an image is formed by light that has passed into the eye via the cornea, aqueous humour, pupil, lens, then the hyaloid and finally the vitreous humour before reaching the retina.
The retina contains photosensitive elements (called rods and cones) that convert the light they detect into nerve impulses that are then sent onto the brain along the optic nerve.

Sclera
The sclera is a tough white sheath around the outside of the eye-ball.
This is the part of the eye that is referred to by the colloquial terms "white of the eye".

Visual Axis
A simple definition of the "visual axis" is "a straight line that passes through both the centre of the pupil and the centre of the fovea". However, there is also a stricter definition (in terms of nodal points) which is important for specialists in optics and related subjects.

Vitreous Humour
The vitreous humour (also known as the "vitreous body") is a jelly-like substance.

Zonules
The zonules (or "zonule fibers") attach the lens to the ciliary muscles.

How the Human Eye Works
In a number of ways, the human eye works much like a digital camera:

Light is focused primarily by the cornea — the clear front surface of the eye, which acts like a camera lens.

The iris of the eye functions like the diaphragm of a camera, controlling the amount of light reaching the back of the eye by automatically adjusting the size of the pupil (aperture).

The eye's crystalline lens is located directly behind the pupil and further focuses light. Through a process called accommodation, this lens helps the eye automatically focus on near and approaching objects, like an autofocus camera lens.

Light focused by the cornea and crystalline lens (and limited by the iris and pupil) then reaches the retina — the light-sensitive inner lining of the back of the eye. The retina acts like an electronic image sensor of a digital camera, converting optical images into electronic signals. The optic nerve then transmits these signals to the visual cortex — the part of the brain that controls our sense of sight.