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The eyes

A close up of a young person's eyes.

The eyes are responsible for four-fifths of all the information our brain receives. Here you can find out a bit more about how they work, common problems that affect vision and the work Sightsavers does to treat and prevent avoidable blindness. You can also find out more about the people whose lives have been changed thanks to donations from people like you.

 

How do eyes work?

(click image to see enlarged version or click here for text alternative)

Graphic of an eye with information about its different parts

The images we see are made up of light reflected from the objects we look at. This light enters the eye through the cornea. Because this part of the eye is curved, it bends the light, creating an upside down image on the retina (this is eventually put the right way up by the brain).

The retina is a complex part of the eye, but only the very back of it is light sensitive. This part of the retina has roughly the area of a 10p coin, and is packed with photosensitive cells called rods and cones.

Cones are the cells responsible for daylight vision. There are three kinds – each responding to a different wavelength of light: red, green and blue. The cones allow us to see images in colour and detail.

Rods are responsible for night vision. They are sensitive to light but not to colour. In darkness, the cones do not function at all.

How do we see an image?

The lens focuses the image. It can do this because it is adjustable – using muscles to change shape and help us focus on objects at different distances. The automatic focusing of the lens is a reflex response and is not controlled by the brain.

Once the image is clearly focused on the sensitive part of the retina, energy in the light that makes up that image creates an electrical signal. Nerve impulses can then carry information about that image to the brain through the optic nerve.

Common eye problems

Short sight (myopia) and long sight (hypermetropia) are common conditions, both caused by the cornea and lens not focusing properly on the retina.

Short sight is where the eyeball is elongated or the lens is too thick, causing the image to focus in front of the retina.

Long sight is where the eyeball is too short or the lens too thin, causing the image to focus behind the retina.

Prescription glasses can help with both long and short sightedness.

There are other sight-threatening eye conditions and eye diseases which are not so easily corrected, including cataract, glaucoma and neglected tropical diseases* like trachoma, river blindness (onchocerciasis), all of which can cause complete blindness if left untreated.

For more than 60 years, Sightsavers has been working with partners to prevent or treat these and other causes of avoidable blindness that affect hundreds of millions of people in developing countries. We do this not only by distributing treatment and carrying out operations to restore sight, but also by strengthening health systems to enable more people to access eye health services, and treatment and operation programmes, into the future.

You can find out more about what we do, or support Sightsavers’ work and help people to see again by making a donation.

*Neglected tropical diseases (NTDs) is the term given to a group of 17 parasitic and bacterial diseases that infect more than one billion people worldwide and are endemic in 149 countries. See the full list of NTDs

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