HOW DO WE ACTUALLY SEE?

 

Vision is god’s gift mankind which is often taken for granted by us. But there are many questions which occur to some of us
– How do we actually see
– How are we able to differentiate between different colors, shapes and forms
– How can we judge the speed of a moving vehicle
– How can we judge the distance of an object from us
– How can we see the objects around us in three dimensions
– Do we really see with our eyes or the brain?

Let us find out how?

EYE AS A CAMERA

Let’s take a look at how each of the components in our eyes is similar to that in a camera. Just like a camera has a lens, a shutter and a film, eye also has a lens (in fact, two lenses, the cornea and the crystalline natural lens, which focus light inside the eye), a shutter (the pupil, which regulates the light entering the eye by cutting off extra light on a bright sunny day and allowing more light to go inside the eye when we are in darkness like in the cinema theater or in the evenings, so that the eye can work well in a wide range of viewing conditions, from dim to very bright light and a film (the retina which acts very much like the imaging sensor chip in a digital camera, on which the image is formed. As with the camera, if the “film” is bad in the eye (i.e. the retina), no matter how good rest of the eye is, we will not get a good quality image or picture.

Therefore, ray of light from any object, travel in the air, reach the eye, and pass through the cornea and the crystalline lens, where they get successively bent or focused to form a clear image on the retina. The image formed on the retina is inverted and two – dimensional.

However, the similarity with the camera ends here. The image that is formed in the camera can be visualized by developing the film or by connecting it to a monitor. However, the optical image formed in the retina does not make us see. There are receptor cells in the retina called rods and cones. They convert the optical image into an electrical impulse, which is carried by nerve fibers (the optic nerve) to a specialized portion of the brain called the visual cortex. It is in the visual cortex, this impulse is perceived as an optical image. Therefore, it is important to realize that while the image is captured by the eye, it is actually the brain which makes us see. So, we can see clearly only when our eyes and visual cortex (in the brain) are healthy too.

The image which is formed in each eye is inverted, and it gets inverted once again in the brain, so that we can see an erect image. Again, the image formed by the eye is two – dimensional. When the images in the two eyes fuse at the level of the brain, we get what is known as binocular vision.

This binocular vision gives us
a. Three dimensional vision
b. Stereoscopic vision i.e. ability to appreciate depth
c. Increased field of vision

The process of image formation is not a static phenomenon and it keeps on happening at a rapid frequency of 60 Hz, modifying the image each time so that we can accurately gauge the distance, speed and direction of a moving object.

HOW DO WE SEE AND IDENTIFY COLORS?

We are able to perceive colours with the help of receptor cells in the retina called cones. The cones have three types of pigments in human responsible for red, green and blue colours and various colours are perceived by the stimulation of three different types of cones in different combinations. This is called tri – chromatic vision. Defects in the retina at the level of cones lead to color blindness.

HOW DO WE SEE IN DARK LIGHT?

There are other types of receptors, the rods that are stimulated in dim light and help us see in the dark. These rods do not contain any pigment. Therefore, we cannot perceive colours in dark of dim light.

WHAT IS A BIONIC EYE?

We have already seen that the eye captures the image but it is the brain which makes us see. People who have lost their eyes but have a functional optic nerve and visual cortex may benefit from the bionic eye, which is an artificial eye under development currently. It consists of a camera attached to a pair of glasses, which transmits high frequency radio signals to the microchip implanted in the retina. The chip transmits impulses to stimulate the retina, which sends impulses to the brain via the optic nerve and which can be interpreted as an image.

SEEN BELOW

1. Schematic representation of an eye as a camera
2. Bionic Eye
3. Schematic representation of a schematic eye