The back cover of the eye. Retina is directly connected to the brain by nerves which transmit impulses in such a way that the Retina is a light sensitive expansion of brain. The back cover or retina consisting of two types of photo receptor cells, the RODS & CONES. Because of the different types of receptor, the eye sees with equal facility under widely different light intensities. The seeing is too efficiently done in light or moderate shadow. Human eye can visualise things around normally under different conditions of illumination. An object seen in a room illuminated by 100 lamps will differ in appearance very little if ninety of the lamps switched off. retina adjusts to the lower lighting level. To the human eye there is no such thing as under or over exposure as in case of a camera. The sensitivity of the Retina changes under different light conditions and it may easily vary from hour to hour and be affected by disease, fatigue and other conditions. Loud sound will decrease Red sensitivity of the eye and increase sensitivity of the Green. The same thing can happen with strong tastes & colors.

The photo receptor cells of retina are of two types ;RODS & CONES.