Eyesight Content from the guide to life, the universe and everything


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An optician puts glasses on a girl.

What does it mean to see? How do we see? How does the mind interpret vision? How does the ability to see affect our lives? What other forms of sight or vision exist? These and other questions will be discussed in this Entry.


Sight is our main access to the world as we know it. A quarter of our brain is devoted to the function of vision, the visual cortex is the most massive system in the human brain. More information enters the human brain through the eye than through any other sense organ. Containing some 170 million rods and cones, 70% of the human body's sensors are found in the eyes. To see is equated with terms such as: to perceive, to understand, to discern, to comprehend and to know. One dictionary defines sight or visual perception as the result of vision, being the ability to detect light and to interpret (see) the consequences of light stimuli. The resulting perception is known as eyesight, sight or vision. We do not always see something even when it is right in front of our eyes. For example, we do not notice our nose, even though it is clearly visible. Thus, to see, in effect can mean, to not see.

How We See

For eyes to function:

  • They need to be open. Often when one finds him or herself unable to see, it is good to make sure that his or her eyes are in fact open, or for that matter that they are still there1.

  • There needs to be some form of light present2. This light needs to be within the range of the visible spectrum. This is the portion of the electromagnetic spectrum which human eyes are capable of seeing. Most human eyes respond to wavelengths between 400 to 700nm3 although some people may be able to perceive wavelengths from 380 to 780nm. The maximum sensitivity of a typical light-adapted eye is at around 555nm which is in the green region.

  • The brain needs to be actively conscious and in full working order. In this respect, it helps not to be dead, unconscious or asleep.

  • It won't work if you are blind. Until recently, there has been no definite cure for blindness. In February of 1999 surgeons performed a ground breaking operation to restore the sight of a man who was suffering from macular degeneration, the most common form of blindness in the western world. Macular degeneration is not a total blindness, but it is a deadening of the retina resulting in peripheral vision only. Other common forms of blindness include glaucoma (increase in eyeball pressure), cataract (lens becomes opaque), common in diabetics, and infectious diseases; 100,000 children each year go blind as a result of measles. For an African or Asian child who has not been immunised, measles can be deadly as the child can go incurably blind in just a few days. There are also infections due to inadequate cleaning of contact lenses, due to Acanthamoeba keratitis. Research into sight restoration is also currently being done with stem cells, and with spinach. For some very useful information about correction of eye defects such as the use of glasses or contact lenses, see Low Vision Aids.

  • It also helps to be reasonably sober. Blurred vision is a common symptom of drunkenness. Alcohol seems to decrease the supply of glucose in the brain. The part of the brain responsible for receiving visual inputs has been found to become especially impaired, consuming 29% less glucose than it should. With less glucose metabolism, it is thought that the cells aren't able to process images properly.

The physical process which takes place in visual perception goes like this:

  • The eye lens focuses light onto the retina.

  • The retina, which consists of a large amount of photo receptors, absorbs light particles, and translates these into electrical nerve signals, which it sends to some layers of cells.

  • The cells receive and retransmit the information just like a chain of people passing on a bucket, until it gets to the brain via the optic nerve.

  • The optic nerves of both eyes converge and cross at the optic chiasm, where the information from both eyes is combined, split and merged to form the picture that we see.

  • The visual cortex will then process the visual image at higher levels.

The Mind's Perception

If you could see the inside of an eye itself, first of all, everything is being projected upside down. Secondly, the muscles which control the eye are always moving, so there is a constant shaking or vibrating which occurs. Not only that, but the picture sent is blurry in all but the middle. The mind does not really see a picture from our eyes, but rather it receives a complex jumble of signals which it has learned to interpret into a clear, steady image. Thus, truth be told, it really is all in the mind. There are many cool mind games and eye tricks, which exploit this.


Much of our ability to see depends on the control of focus. To focus is to converge both eyes to a central point of interest; to concentrate on something. Without this we would merely see a vast mishmash of colours and shapes, with no way of differentiating what is what. Humans can distinguish over 10,000 different colours. This is where it becomes very useful to have two or more eyes. Because each eye projects a separate, distinct image, the brain overlaps the two images to create a three dimensional picture. A person who only has one eye must constantly struggle to define depth which is the ability to judge objects that are nearer or farther than others. When the eyes focus on an object they block out all other objects in the field of view. Like many other predatory animals, we humans have what we call stereoscopic vision. This is vital for clear sight to work in three dimensions, for judging distances and for the brain to recognise and process information. Stereoscopic vision occurs chiefly in predators as it is very useful for hunting and tracking down prey.

Animals' Vision

Many species can perceive wavelengths outside of our own range of light visibility, and that's not all...

  • Bees and many other insects can detect ultraviolet light which is useful to home in on flowers.

  • At the other end of the spectrum, some snakes are sensitive to infrared which they use to detect the presence of warm-blooded animals.

  • The eyes of a goldfish are so sensitive, they can see the infrared beams that control our televisions and videos.

  • Of the estimated 900,000 or so animal species on the planet, half of these are nocturnal, and have extremely sensitive eyes. Many use infrared surveillance.

  • Apparently cats can see in colour, but only between the low to mid light wave spectrum. Thus they see blues and greens, but no reds, oranges or browns. These colours would appear to them as shades of grey or purple. Cats' eyes catch 50% more light than ours and are eight times more sensitive than ours at night. The human eye is so sensitive that it can detect a lighted candle at a distance of 1.6 km (one mile).

  • Owls use the reflective 'shininess' in their eyes to optimise on capture of what little light remains during darkness. Owls have a sensitivity to low light intensities 50 to 100 times greater than that of unaided human night vision.

  • 77% of the animal kingdom consists of creatures such as insects and crustaceans which have compound eyes. Compound eyes have poor resolution, but they can see things at a very wide view angle and have the ability to detect rapid movement.

  • Birds have the greatest range of Dioptres of all animals. A Dioptre is a measurement of focal range, or distance that an animal can focus on. A human has around 14 Dioptres, while a diving bird has 50. That is why a hawk can spot a mouse from 1.5km high.

  • Depending on where the eyes are positioned on the head, different animals will have a different field of view. A human field view is about 160 to 208 degrees. A dog's is about 280 degrees, whereas a hare can see a complete view of 360 degrees.

  • The chameleon famously has two eyes that work independently of one another. Thus it can be looking up with one eye, and down with the other. The reason for this is that rather than being sunk into eye sockets, its eyes are mounted on little turrets. But even so, a chameleon’s eyes can see colour just like we do. Chameleons, as well as some other kinds of lizards, have a rudimentary third eye which can detect violet and blue light.

  • Bats are not blind as the myth suggests. Many species, particularly fruit bats, can see at least as well as we can. But insectivorous bats also perceive the world by a totally different means - that is echolocation or high frequency sound. The main difference between this and sight is that sight is the detection of light that exists in the environment, whereas a bat is picking up sound frequencies which have been generated by the bat itself. The end result for the bat is the perception of distance, speed, and direction of movement; the main quality of an object determinable by eyesight that is not determinable by ultra-sound is colour. Much in a similar way that bats use echoes, dolphins also use sonar4.

1Sadly, in many parts of the world, it is all too common to suffer loss of eyes as a result of antipersonnel land mines - which are designed to wound - not kill. About 80 million land mines are still buried worldwide. About one third of land mine victims are children. Loss of eyes is also common from bomb blasts and shooting accidents.2If in doubt, look for the light switch, or make sure that you always carry a lighter, key-ring torch or mobile phone. These instruments are the modern answer for trying to find the keyhole in the dark.3nm = nanometer. A meter is the length of a path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second.4We must be clear that these things are not classified as sight, they are advanced forms of hearing - which come close to the higher level of viewing the world - but they are not sight itself.

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