Disclaimer: Nothing in this article is to be construed as medical advice, nor is it intended to replace the recommendations of a medical professional. For any specific questions about your eyes, please see your eye care practitioner.
Sense is a method of perception, helping us to get information about what is going on around us. Human has five senses: taste, sight, touch, smell and sounding. Human eye is an amazing organ which reacts to light and gives us sense of sight. Eyes capture light and transform it into electrical impulses to our brain and let us interpret the surrounding environment using light in the visible spectrum reflected by the objects in the environment. However, not eyes of all creatures evolved and structured in the same way, for example, human vision is different from bird vision or bug vision.
Figure 1: Eye Structure of a Human Eye
As you can see from Fig 1, light first pass though and is refracted by the cornea, the transparent front of the eye. Refracted light enters the pupil, an adjustable hole controlled by iris, and is then focused on the retina by the lens. Light-sensitive nerve cells in the retina like rods (for brightness) and cones (for color) react to the light and turn light into electrical signals to our brain. They interact with each other and send messages to the brain. The rods and cones enable vision.
With two eyes, our brain receives two slightly different light signals from different angles. The brain combines these two images into a three dimensional image, telling us how something is far away. We called it depth perception.
Cornea is the clear and transparent front part of our eye. Most of us know it refract light and contributes most of the focusing power of our eyes. Astigmatism usually is caused by an irregularly shaped Cornea, which forms multiple focuses on the retina, resulting in blurred or distorted vision. We called it corneal astigmatism. Squinting is the most common symptom of astigmatism, as people squint to see better.
Another function of Cornea is UV filter of our eyes. It absorbs ultraviolet (“UV”) radiation and protect the internal part of eye structure, such as retina. Studies found that the corneal layers absorb the UV rays from 240-400 nm and being important in preventing damage of our internal eye structure by UVB.
Even though cornea protects the internal eye structure from UV radiation, it can be damaged easily by over-exposure to the UV radiation from the sun or from other sources of UV light, such as tanning lamp, flash light. Such damage to cornea can be considered as the sunburn of cornea. It makes our eyes in pain, at worst loss of vision.
Pupil and Iris
Figure 2: Pupil and Iris
The pupil is the small black hole in the center of the iris of the eye that allows light to reach retina. It looks like a black hole or spot in our eyes, because light, passing through the pupil, is eventually absorbed and turned into electrical signal by the retina. Human pupil is circular-shaped, but some animals have vertical pupils, whereas others have horizontal. An interesting study found that the pupil shape may relate to the foraging mode of the animals. For example, herbivorous animals are very likely to have horizontal pupils and ambush animals have vertical pupils.
Take camera as an analogy, the relationship between pupil and iris just like the aperture and aperture stop. The iris changes the size of the pupil to let more or less light in to prevent damage to the eyes by over-dosage of light. The pupil gets wider in the dark but narrower in the bright. Normally, the size of the pupil is controlled by the iris automatically in response to the brightness of the environment. However, medical provider may us special eye drops to make your pupil dilating (i.e., being wider) for examination of retina, as only a limited view can be observed through an undilated pupil. An eye protection is normally required after such eye examination in order to prevent over-exposure of light to the eyes.
The lens is another transparent structure in the eye, apart from the cornea. It is located behind the pupil and suspended by the ciliary muscles surrounding the lens. The ciliary muscles change the curvature of the lens, just like the iris adjust the size of pupil. The adjustment of curvature of the lens is called accommodation. Accommodation enable us to maintain a clear image on an object as its distance change, for a running object. Cornea and lens are responsible for the optical power of our eye. A diopter, a unit of measurement of the optical power of a curved mirror or lens, is equal to the reciprocal of the focal length in meter, in short, it is a unit of reciprocal length. The young men normally have the best vision, approximately 15 diopters, i.e., 1/15 and can change focus from the distant to as near as less than 6.6 cm (i.e., 1/15 meter). Accommodation, the focal power of the eyes, declines dramatically with age, and most of people have approximately less than 2 diopters at the age of 45 to 50 years. Accommodation decreases to about 1 diopter by the time a person reaches 70 years.
The natural crystalline lens of the eye may not be perfectly symmetrical front and back, such mis-shaped lens can lead to astigmatism. This is called lenticular astigmatism, which is different from the more common corneal astigmatism.
Similar to cornea, lens of the eye is also responsible to filter UV radiations in the wavelength between 300 to 400 nm, i.e., mainly UVA and protect the retina away from such UV radiations. Although lens of the eye absorb UV rays, UV rays would contribute to the cause of Cataract, which is a clouding that develops in the crystalline lens, obstructs the passage of light and leads to a decrease in vision. Surgical operation to remove the cloudy lens and replace it with an artificial lens is the only way to get rid of cataract. Cataract is generally considered as aging-associated diseases, but studies shown how UV rays may contribute to cataract. Prevention is better than cure - wearing UV blocking sunglasses to keep eyes away from harmful UV rays. Cataract also happens to other mammals, like dogs.
Figure 3: The Old Chihuahua with Cataract
Retina is display used in many products of Apple, like iPhone, iPad, iPad Pro, Macbook Air, Macbook pro and iMac. Haha...ha, it is not the retina we are referring to. The retina of the eye is a very thin layer of tissue in which consists of our photosensitive cells and optical nerve cells. Photosensitive cells in the retina is to receive light that the lens has focused, convert the light into electrical neural signals by the optical nerve cells, and send these signals to the brain for visual recognition.
Rods and Cones
Photosensitive cells in the retina allow are light sensitive cells, responsible for detecting qualities of light such as color and light intensity. Two types of photosensitive cells in our eyes help us to differentiate colors and capture light in various light intensity conditions: cones and rods. In adult human eyes, the entire retina contains about 6 million cones and 120 million rods. Cones provide us with color vision, but they require relatively more light in order to generate signals. Humans have three different types of cone cell, which react to different wavelengths of visible light and generate three different and distinct signals. They are S-Cone, M-Cone and L-Cone, which react to light of short wavelengths (i.e., S-Cone response to blue), medium wavelengths (i.e., M-Cone responses to green), and long wavelengths (i.e., L-Cone responses to red). Therefore, we said humans have trichromatic vision. Color blindness occurs when one or more of the cone types are faulty. The rods are narrower than the cones and distributed differently across the retina. Unlike cones, rods are extremely sensitive to light. At a very dim-light environment, our vision is based solely on the rods. The faulty of rods cause to night blindness.
Other animals may view differently from us, as they have more Photosensitive cells in their retina. Some birds have four types of cones rather than the three we have. The additional cone cell of birds reacts to ultraviolet, which is invisible to humans. The exact number of cones varies in each bird species but is typically higher than humans. These explains why bird is considered to have a better vision than us.
Due to the retina's vital role in converting light into biological neural signal, damage to it can cause permanent vision loss, retinal detachment, where the retina is abnormally pulls away from its normal position, can separate the retinal cells from the layer of blood vessels that transport oxygen and nourishment to the retina. It is a very emergency situation, the longer the retina detached from the blood vessel layer, the greater the risk of permanent vision loss of the affected eye.
 Lajos Kolozsvári, Antal Nógrádi, Béla Hopp, Zsolt Bor; UV Absorbance of the Human Cornea in the 240- to 400-nm Range. Invest. Ophthalmol. Vis. Sci. 2002;43(7):2165-2168.
 Martin S. Banks, William W. Sprague, Jürgen Schmoll, Jared A. Q. Parnell and Gordon D. Love, Why do animal eyes have pupils of different shapes?
Science Advances 07 Aug 2015: Vol. 1, no. 7, e1500391 DOI:10.1126/sciadv.1500391.
 Duane A: Studies in monocular and binocolar accommodation with their clinical applications. Am J Ophthalmol 5:865, 1922.
(Courtesy: National Eye Institute, National Institutes of Health (NEI/NIH))