The light waves reach the eye and are converted into electrochemical stimuli and, thanks to the optic nerve, are transmitted to the brain, which - as in the case of sound stimuli - "decodes" and interprets them as three-dimensional images.
The eye is made up of an outer membrane called sclerotic (which we could compare to a camera lens), the front of which is the hornsto.
There is a second membrane, the choroid, the front of which is colored, is called iris and has a central hole called pupil; depending on the amount of light present outside, the iris narrows or widens to let more or less light into the pupil.
Returning to the comparison with the camera, the choroid could be represented by the camera obscura and the iris by the diaphragm.
The eye, too, needs to focus the images and does so thanks to a biconvex lens placed behind the pupil called crystalline, which performs this task by changing its curvature.
But the camera also has the film! In the eye this task is performed by a very thin membrane, the retina, which is made up of cells with the characteristic of being sensitive to light (i.e. photosensitive). The power of accommodation is a parameter that represents the ability of the lens to modify its curvature in order to focus on an object at any distance from the eye; if the image is located at a distance of less than 100 meters, the lens increases by thickness to be able to concentrate the light rays on the retina since the latter reach divergent to the eye. While, when the image is at a distance greater than 100 meters, the lens easily concentrates the light rays on the retina as these reach almost parallel to the eye.
Fun fact: hawks have excellent eyesight! Hence the saying "hawk view"! These birds, in fact, possess a muscle which causes the eye's accommodation power to be faster than that of man.
But who is responsible for transforming the image into electrochemical stimuli that are then transmitted to the brain? The light that reaches the back of the eye is converted into bioelectrical signals that reach the brain: there are chemicals that change when they are hit from the light; these substances are contained in the cones and rods (called photoreceptors); the cones are used for color vision and are mainly found in the central area of the retina. There are about 6 million cones per eye and there are three different types: for green, for yellow and for red. The rods, on the other hand, are about 120 million and are used for vision in the dark; they are mainly present in the peripheral area of the retina. The pigment of the rods is the rhodopsin, which consists of the retinene (a group of atoms that absorb light called chromophores) and from the "opsin which is a protein that facilitates the chemical reaction.
If the light affects the retinene its structure changes: the rotation of the terminal chain connected to the opsin is induced (it passes from the cis form to the trans form): the rhodopsin molecule is transformed into metarodopsin I, first, and then in metarhodopsin II; thus, electrochemical impulses are produced in the nerve cells of the retina.
With a sudden glare or when the environment in which we find ourselves is very bright, or if there is a violent change in brightness, the eyes react quickly in order to reduce the amount of light that reaches the retina by narrowing the pupils and squinting the eyelids; but the vision has been reduced anyway, since the rhodopsin has been transformed and the impulses sent to the optic nerve are weaker; for this it takes a few seconds to restore the optimal function of the photoreceptors and, if in cases like these if you are driving a vehicle, it is advisable to slow down !!
On the other hand, passing from light to dark, also in this case the eyes adapt to the new situation: the pupils dilate to let in as much light as possible and the photosensitive pigment of rhodopsin is produced in the rods; unfortunately, the formation of rhodopsin takes about 10/20 minutes and it is only after this time that the eye is able to produce the impulses that allow the individual to perceive the little light present. Even in this situation, you must slow down if you are driving a vehicle.
Therefore, following the alteration of the aforementioned substances, caused by the presence or absence of light, the impulses are generated which, through the optic nerve, reach the brain. To see well, not only two good eyes are needed ... it takes a brain! !
The amplitude of the visual field decreases if the speed is increased; and this must be taken into account when driving a vehicle, as well as the fact that only one eye is not able to accurately detect the actual consistency of an object but only the simultaneous functioning of the two retinas of two eyes, allows to understand the correct relief of the objects and the distance from the observer.
When driving a vehicle on the road, visibility also depends on the visibility distance, which is a parameter given by the sum of the space needed to maneuver the vehicle and the space covered during the driver's reaction time.
The average time taken by the visual stimulus to reach the brain and be decoded is between 0.7 and 1.3 seconds, which therefore corresponds to the reaction time in front of an obstacle. Alcohol alters eye movements and consequently lengthens reaction time by up to 2.5 seconds.
Other articles on "Eye, Sight and Road Safety"
- Ear, hearing and road safety
- BAC or BAC
- Effects of alcohol
- Sleep and road safety
