When looking at an object, the light reflecting from it enters the eye. This light enters the eye through the cornea which is the clear layer of tissue in front of the eye. The cornea bends the light and passes through the iris, which changes the size of the pupil depending on the brightness of the light. Once through the pupil, the light will enter the lens which focuses the light through the vitreous humour on to the retinal wall. The retina’s surface comprises of photoreceptors (light-sensitive neurons) that translate the light signal into an electric one and sends the information along the optic nerve to the brain.
In a normal eye, the cornea and crystalline lens focus the light exactly on to a small area of the retina, producing a sharp image.
Short-sightedness (myopia) occurs when the eyeball is too long. This causes light rays entering through the front of the eye to focus in front of the retina, rather than on it. It can also be due an abnormally shaped cornea or lens compared to the length of the eyeball.Myopia
Long-sightedness (hyperopia) occurs when the eyeball is too small. This causes light rays entering through the front of the eye to focus behind the retina, instead of on it. It can also be due to the cornea or lens curving abnormally compared to the length of the eyeball.Hyperopia
Astigmatism is caused by the light being bent incorrectly by a misshapen cornea, which is rugby ball shaped instead of spherical. This causes a distorted and blurred image, often characterised by ‘ghosting’ especially when looking at letters on a screen.Astigmatism
Presbyopia is part of the eye’s natural ageing process. It happens because the lens inside the eye (crystalline lens) starts to become less flexible and elastic with age.Presbyopia