Human eye facts > How the human eye works

The sight is the most important sense out of all five because it enables us to receive the information from the environment. The ability of the human eye to receive inputs from the outside is enabled by light refracting areas and the retina. We can simply say that the rays reflected from the object, which we are looking at, have to pass through four light refracting areas, the cornea, the aqueous humour, the eye lens and the vitreous body which form the optical system of the eye and create a real diminished image at one point on the retina which we then perceive.

How the human eye works?

In a simplified way we can liken the human eye to a camera, where the cornea and the lens fulfill the role of objective, the pupil operates as a diaphragm and the retina substitutes a sensitive film. The rays of light from shining or illuminated object have to fall on the eye, pass through the optical system and then they are directed towards the retina.

A visible image can be created only by the rays of the wavelength between 400-760 nm. Directing of the rays of light towards one point on the retina is called optical power or refractive power and is determined by the incurvation of the light refracting areas and the refractive index. The refractive power is measured in diopters.

The cornea and the eye lens are the most important refractive areas of the human eye. The cornea has the refractive power of around 43 diopters and the lens has the power of 20 diopters. The refractive power of the human eye is not always the same but it can change depending on whether we are looking into the distance or close to us. It is determined by the elasticity of the eye lens which permits the change of its incurvation and thus also the change of refractive power. This activity is called the accommodation (adaptation) and it permits us to see sharply the objects which are far away or close to us. When looking at something which is close to us, the eye has to adapt, the lens becomes more convex therefore increasing its refractive power. On the contrary, when looking into the distance (the objects in the distance of more than 5-6 meters from the eye) the lens becomes more concave and decreases its refractive power. For example, when reading from the distance of 25 cm, the lens has to increase its refractive power by 4 diopters, when reading from 33 cm by 3 diopters. All of this happens automatically without our will. As people get older and the lens becomes less flexible, it is becoming increasingly difficult to perceive the objects which are close to it. Therefore elderly people have to use glasses when reading or working with something which is close to them.

The important condition for the image to be created on the retina is a proper reciprocal ratio between the refractive power of the eye and its length. This relation is called the refraction. If the ratio is balanced then the rays of light refract towards one point on the retina. The human eye like this is called stigmatic. If the ratio is not balanced as a result of greater eye length or increased refractive power, the focal point lies in front of the retina which results in unsharpened vision of the objects in the environment. Such a disorder is called short-sightedness. If the eye length is too short or the refractive power is decreased, the focal point lies behind the retina. Such a disorder is called long-sightedness.