Mechanism of light perception :

The human eye has two functional parts – Dioptric or Focussing

part and Receptor part.

(i) Focussing part : It consists of conjunctiva, cornea, aqueous humour lens and vitreous humour. These part are transparent and act as lenses. They refract the light rays passing through the eye to bring them to a focus on the retina. Cornea causes maximum refraction . Cornea places the image approximately on the retina. The lens effects fine adjustment and brings the image into a sharp focus.

(ii) Receptor part : It comprises the retina. Inverted and smaller image formed on the retina . It converts the energy of specific wave lengths of light into action potential in nerve fibre.

(b) Pathway of sensory impulses from eye to brain : The nerve impulses generated in the retina of the eye in response to light follow a definitive path and terminates in visual cortex in each optic lobe which act

as primary visual center.

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How human eye work for vision?

Biochemistry of eye

The receptor cells of eye are called photoreceptor or visual cells. They are of two types – Rod cells and Cone.

How human eye work for vision?

(a) Rod cells :

The rod cells contain a purplish pigment called visual purple or rhodopsin.

They function in dim light and at night.

They produce poorly defined images.

Bright light splits rhodopsin into a lipoprotein scotopsin and a carotenoid pigment retinal (retinine) a process called bleaching.

The spiliting of rhodopsin depolarizes the rod cell and it releases a neurotransmitter, passing the nerve impulse via bipolar neuron and ganglion cells to the optic nerve.

Scotopsin and retinal unite to form rhodopsin in dark.

This process is called ‘dark adaptation’.

It makes the rods functional.

Rhodopsin takes some time to reform.

This is why on entering a dark room at daytime or on coming out of a well lighted room at night we feel blind for a while,

When we go from darkness into bright light, we feel difficulty in seeing properly for a moment till rhodopsin is bleached and cones become functional.

(b) Cone cells

Cones contain iodopsin which is visual violet and made up of photopsin + retinal.

Cone cells are of 3 types.

1. Erythrolobe (775 nm sensitive tored), 2. cyanolabe (430 nm sensitive to blue) and 3. chlorolabe (sensitive to green 535 nm).

White light is perceived when all cone types are simultaneously stimulated by equal amounts of coloured light than sensation.

Diurnal animals can see during day light (Photopic vision).

Diurnal animals can perceive colour.

They can not perceive colour in dark. Such animals have more cones in their eye than rods.

How human eye work for vision?

Accommodation :

Before focusing on the retina.

Light passes through many refractive surfaces .

An inverted and true image forms on retina.

The main sites of refraction are

1.cornea

2. aqueous humor

3. iris

4.lens (ciliary body can alter the position of lens : accommodation)

5. posterior chamber (= vitreous humor)

6. retina ( in fovea).

The refractive index of the eye varies from 59 diopter (when the lens is at rest) to about 71 diopter (when lens is bulging in maximum accomodation).

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The accommodation reflex occurs when the eye changes its focus from a far away object to nearer one.

The change in strength of the lens provides the physiological basis of accommodation.

Radial and circular muscle fibres of ciliary muscles play an important role.

They contract reflexively (parasympathetic control) and increase lens strength.

The pupil constricts. This facilitates increase in sharpness of image.

Ageing causes loss of accomodation.