We are going to discuss how the eye can distinguish between light and dark at pigment level.
A discussion on what happens in the eye under different light intensities.
Rods and cones, the photoreceptors in our retina, are responsible for detecting light. How? With the use of light sensitive pigments.
Each pigment is made of a protein called opsin and a chromophore called retinene, (a variant of Vitamin A) .
=opsin + chromophore
The following cycle of events takes place in rhodopsin when light strikes on rod cells in retina:
Bleaching of iodopsin in cones is similar to rhodopsin in rods. But more light is needed to cause an action potential to be fired in cones. (i.e. Threshold intensity for cones is higher than rods.)
In other words, rods are mainly used for dim light vision, cones for bright light vision.
The use of both rods and cones to see is called the Duplicity Theory.
Vitamin A myth: You must have heard that if we do not have enough Vit. A in our body, we would have night blindness. Take note: 1) Excess Vitamin intake can become toxic and harm our body. 2) Lack of Vit A is not the only cause of poor vision. Vit B is also essential for normal functioning of the retina.
We need a period of time before our eye can adapt to new light conditions and see properly. This is called Adaptation.
It takes about 20 minutes for enough rhodopsin to reform for us to see properly.
When we move from a dark room to a brightly lit room, we feel uncomfortable from the glare. But after some time, the visual threshold in cones increases relative to the generator potential. Cones is less stimulated, and we will see better. This takes about 5 minutes.
Pilots wear red goggles in strong light to be less influenced by changes in light intensity.
Reason: red light stimulates rhodopsin (in rods) less, i.e. less rhodopsin is bleached; less time is needed for the eye to become dark-adapted.
Back to main Physiology page