For example, a red sclera might suggest that your eyes and tired or dry , while a yellow-tinted sclera could indicate liver problems. Iris: The iris is the coloured part of your eye and is what actually controls the size of the pupil. This means that it regulates how much light gets into the eye. This iris is made from connective tissue and muscle surrounding the pupil, and its structure, pattern and colour are just as unique as your fingerprint!
Cornea: The cornea is the clear surface at the front of your eye, allowing light to enter the eye. It directly covers your iris and pupil, providing a layer of protection.
The cornea is what we operate on for laser eye surgery procedures , as it is imperfections in the curve of your cornea that create an eye prescription, requiring you to need glasses. The smoother the surface of your cornea is, the better your vision will be.
Lens: The lens is located behind your iris and is the part of the eye which provides focus. The lens can change shape to alter the focal distance of the eye, focusing light rays that pass through it to hit the retina at the right angle. As you get older, a build-up of protein in the eye can mean that the lens becomes cloudy. This is called a cataract. Thankfully, your lens is easily removable and can be replaced with an artificial clear lens to provide good vision again. Aqueous humour: The aqueous humour is a watery fluid that your eyes constantly produce in order to maintain good eye pressure and nourish your cornea.
This keeps your eyes healthy and, in turn, contributes to good vision. Ciliary muscle: The ciliary muscle is the part of the eye that actually changes the shape of the lens, allowing it to focus on different distances. Medial rectus muscle: There are six extraocular movement muscles in your eye medial rectus, lateral rectus, superior oblique, superior rectus, inferior rectus, and the inferior oblique and the medical rectus is the largest of them.
It moves the pupil closer to the midline of your body towards your nose and makes sure that the eye is aligned correctly. If there are problems with the medial rectus, it can lead to strabismus. Lateral rectus muscle: This is the muscle which is responsible for lateral — or sideways — movement of the eye, particularly movements away from the midline. As soon as the eye moves, it re-adjusts its exposure, both chemically and geometrically, by adjusting the iris which regulates the size of the pupil.
Initial dark adaptation takes place in approximately four seconds of profound, uninterrupted darkness; full adaptation, through adjustments in retinal chemistry, is mostly complete in thirty minutes.
Hence, a dynamic contrast ratio of about 1,, about 20 f-stops is possible. The process is nonlinear and multifaceted, so an interruption by light starts the adaptation process over again. Full adaptation is dependent on good blood flow thus dark adaptation may be hampered by poor circulation, and vasoconstrictors like tobacco. The eye includes a lens not dissimilar to lenses found in optical instruments such as cameras.
The same principles can be applied. The pupil of the human eye is its aperture. The iris is the diaphragm that serves as the aperture stop. Refraction in the cornea causes the effective aperture the entrance pupil to differ slightly from the physical pupil diameter. In order for the human eye to see clearly, the image needs to be formed directly on the retina; if it is not, the image is blurry.
It allows light perception, color vision, and depth perception, but not all eyes are perfect. In the middle of the iris is the pupil, which is the black dot that changes size. The cornea covers these elements, but it is transparent. The fundus is on the opposite of the pupil, but inside the eye and cannot be seen without special instruments.
The different parts of the eye have different refractive indexes, and this is what bends the rays to form an image. The image passes through several layers of the eye, but this happens in a way very similar to that of a convex lens. For the vision to be clear, the image has to be formed directly on the retina. The focus needs to be changed, much like a camera, depending on the distance and size of the object.
This changes the focal length. When you focus on something, you squeeze or relax these muscles. Nearsightedness, or myopia is a vision defect that occurs when the focus of the image is in front of the retina. This is shown in. Close objects are seen fine, but distant objects are blurry. This can be corrected by placing diverging lenses in front of the eye.
This will cause the light rays to spread out before they enter the eye. Near Sighted Vision : This occurs when the image is formed before the retina. Farsightedness, or hyperopia, is a vision defect that occurs when the focus of the image is behind the retina. Distant objects are seen fine, but closer objects are blurry. This can be corrected by placing converging lenses in front of the eye. This will cause the light rays to slightly converge together before they enter the eye.
Far Sighted Vision : This occurs when the image is formed behind the retina. Privacy Policy. Skip to main content. Vision and Optical Instruments. Search for:. The Human Eye. The Human Eye The human eye is an organ that reacts with light and allows light perception, color vision and depth perception.
Learning Objectives Identify parts of human eye and their functions. Key Takeaways Key Points The eye is made up of a number of parts, including the iris, pupil, cornea, and retina.
The eye has six muscles which control the eye movement, all providing different tension and torque. The eye works a lot like a camera, the pupil provides the f-stop, the iris the aperture stop, the cornea resembles a lens.
In its natural state, the lens looks like an elongated sphere — a shape known as ellipsoid — that resembles a deflated ball. The average lens size in adults is approximately 10 mm across and 4 mm from front to back.
The lens is made up almost entirely of proteins. The tissue is transparent, which allows light to easily enter the eye. The primary function of the lens is to bend and focus light to create a sharp image. To do that, the lens uses the help of ciliary muscles to stretch and thin out when focusing on distant objects, or to shrink and thicken when focusing on near objects. When light enters the eye, the lens will bend and focus incoming light directly on the retina, which is how the clearest possible image is produced.
The crystalline lens projects a focused image on the retina. However, the initial image projected is inverted either upside down or reversed.
When the image is sent to the brain via the optic nerve , the brain will flip the image back to normal. The ciliary body is critical for the lens to function correctly. While the ciliary muscles allow the lens to change shape to focus, the lens itself is kept in place by little fibers that are connected to the ciliary body — these are called zonular fibers, or zonules.
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