The Human Eye

Photoreceptors in the retina

External and Internal Structure of the Eye

connected to nerve endings from the optic nerve

Rods and cones are photoreceptors

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Cones

Rods

enables us to see colours in bright light

three types of cones: red, blue & green cones

do not work well in dim light

enable us to see in dim light in black & white

sensitive to light of low intensity because they contain a pigment called visual purple

when the eye is exposed to bright light, all the visual purple is bleached

upon movement from a brightly-lit room to a dimly-lit room, visual purple needs to be formed again and it takes some time

formation of visual purple requires Vitamin A.

a person deficient in Vitamin A maybe unable to see in dim light & is said to suffer from night blindness.

Pupil Reflex

controls the amount of light entering the eye

by altering the diameter of the pupil

Size of pupil is controlled by 2 involuntary muscles in iris; radial & circular muscles

stimulus -> receptor (retina) -> sensory neuron in optic nerve -> brain -> motor neuron -> effector (iris)

Characteristics of image formed in the eye

Accommodation

inverted

laterally inverted

diminished

adjustment of the lens of the eye so that clear images of objects at different distances are formed on the retina