Eye (biophysics)/Principle of vision

The eye is a combined optical system with a variable focal length. The resulting image is reduced, inverted and real.


 * when the beam enters the eye, the light passes through the cornea and ocular fluid
 * variable opening in the iris controls the intensity of incident light
 * after passing through the lens (the largest refractive index of all parts of the eye) the light spreads with the vitreous (smallest refractive index)
 * the image formed on the retina, in which there are two types of cells:
 * rods (perceive light intensity)
 * cones (perceive colors)
 * Here are the endings of the of the optic nerve, that carry sensations to the brain.


 * two main points are used to orient the eye doctor on the retina:
 * yellow spot (macula lutea) – the place of greatest eye sensitivity
 * blind spot (discus nervi optici; papilla nervi optici) – place where the optic nerve enters the eye, i.e. a place where there are no sticks or suppositories


 * the emergence of a sharp image is made possible by accommodating ability of the lens, which is held by the ciliary muscle, which changes the curvature of the optical surfaces as needed (at a long distance, less curvature and vice versa):
 * when looking at a far point (infinity) the accommodation of the eye is minimal
 * when looking at the near point (up to 15 cm), the accommodation of the eye is maximum


 * Conventional visual distance = 25 cm.

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