Working Principle of Eye

The camera and eye work on the same basic principles.

The amount of entering light according to intensity is controlled by the changing diameter or aperture of pupil.

Focussing images of the object by means of a lens system.

Registering the image on a sensitive surface by refraction or convergence of light.

Processing of the captured images to produce a pattern that can be seen.

Refraction (or Focussing)

Tear conjunctiva cornea aqueous humor lens vitreous humor refract the light rays to converge upon retina.

Cornea refracts the most and lens the next.

Lens focuses the ray exactly upon retina to form image. According to distance of the object accommodation is made by changing the curvature (convexity) of lens by ciliary muscles.

When ciliary muscle is relaxed the eye is at rest and focuses for distant object. But for focussing the near objects ciliary muscle contracts to increase the convexity of the lens. Frog has poor accommodation ability, in water it is hypermetropic while on land it is myopic. However, a slight accommodation can be made by shifting the lens forward and backward like camera.

The inverted (virtual) image is formed upon retina, and the normal vision is termed emmetropic.

In human each eye has the visual field of about 170°. The visual fields overlap considerably which helps us in judging the relative position of objects in our visual field, this is depth- perception or stereoscopic vision.

The normal range of vision in human eye is from 30 cm to 6 meters.

Chemistry of Vision

Light ray bleaches rhodopsin into a protein opsin (scotopsin) and a pigment retinene (Vit. A). Being aldehyde retinene is also called as retinal. In dim light these recombine to form rhodopsin. This cyclical process keeps going on and the impulse generated in the rod cells is transmitted to brain. In bright light it gets permanently bleached, therefore, when we enter the room from bright light (outside) it causes blinding effect for a short while (10-20 sec) till the opsin and retinene recombine.

Periodic blinking of eye perhaps provides this moment for the resynthesis of rhodopsin.

Similar process occurs in the cone cells in bright light. The opsin of cones is photopsin only slightly different from scotopsin.

Common defects of vision

Short-sightedness (or Myopia):

Near vision is clear, but distant vision blurred,due to longer antero-posterior diameter of eye ball; distant object not focused upon retina but in front of it; corrected by use of concave lens.

Long sightedness (or Hyperopia or Hypermetropia)

Distant vision clear but near vision blurred, due to shorter antero-posterior diameter of eye ball, near objects not focused upon retina but behind it; corrected by use of convex lens.

Presbyopia

A hyperopic condition due to loss of elasticity of lens in old age, ciliary muscle fails to increase its convexity (the diopteric power), corrected by using convex lens (reading glass).

Astigmatism

Due to irregular curvature of the corneal surface some light rays bend more than others which results in improper focusing and distorted images on retina, corrected by using cylindrical lens.