Introduction:

The sense of vision is one of our most valued possessions. This lets us to enjoy the splendor of nature, excites our thinking and enriches our lives in numerous ways. We become aware of the infinite variety of objects around us, specially their shapes, colors, textures, motion, and so on, only due to our capability to observe them. However have you ever thought: What makes us to see? It all starts with eyes though as well based on what happens behind the eye. Each and every object viewed is seen by light. The eye responds to illumination. We all are familiar that all living species - from one celled amoeba to the great bald eagle - have developed special methods for responding to light.

Human perception of light, that is, vision, is a more developed procedure. It occurs almost spontaneously devoid of anyone, other than the perceiver, knowing what is happening. Perception of light comprises the formation of sharp images (that is, in the visual apparatus) and their interpretation. Vision starts in the eye, however light is sensed through the brain. However, what we see is the world made by our visual apparatus within our head. Therefore we can state that vision comprises a mix of physical and physiological phenomena.

Human Vision:

Vision comprises a mix of physical phenomena and physiological methods. We can recognize how the image of an object is made within the eye purely in terms of physical principles and processes. However from image formation to its perception by the brain, the procedure is physiological.

Human vision as well consists of a rich relationship by other senses. However, all our five senses cooperate and augment one other.

Our eyes are extremely versatile. They have a staggering degree of adaptability and accuracy. They are able of very fast movement. That is why we can instantly shift the focus from a book in hand to a far-away star, familiarize ourselves to bright or dim light and differentiate colors, scan space, estimate distance, size and direction of movement.

Physiologists state that the human eye is an image-making device. (In an approach, the human eye consists of striking similarities to a camera of automatic intensity and focal control.) To recognize the details of the method of vision, some knowledge of the visual apparatus is essential.

Viewing Apparatus: The Eye

Our eyes, as we recognize, are positioned in the bony sockets and are cushioned in fatty connective tissue. The adult human eye measures around 1.5 cm in diameter. The diagram of the human eye is as shown below.

The sclera or white of the eyeball is an opaque, fibro-elastic capsule. This is quite tough and provides shape to the eyeball, protects its inner portions and withstands the intraocular pressure in the eye. The muscle fibres that control eyeball movement are inserted on the sclera. The cornea is a strong curved front membrane which covers the iris, the colored circular curtain in the eye. The cornea works as transparent window to the eye and is the main converging element.

The cornea is followed through a chamber filled by a transparent watery liquid, the aqueous humor that is generated constantly in the eye.

This is mostly responsible for the maintenance of intraocular pressure. Alongside this, aqueous humor is the merely link between the circulatory system and the eye-lens or cornea. The intraocular pressure maintains the shape of the eye assists to keep the retina smoothly applied to the choroid and makes clear images. Close to the rear of this chamber is the iris. The iris is opaque however consists of a small central hole (that is, aperture), termed as pupil. In general observation, the pupil seems more similar to a black solid screen. This is due to the reason that behind the opening is the dark interior of the eye. The size of the pupil in normal eye is around 2 mm. The light enters the eye-ball via this area. The iris is hanged between the cornea and the lens.

The main function of the iris is to control the intensity of light entering the eye-ball. If the light is bright, the iris contracts and the size of the pupil reduce and vice-versa.

Thread-like suspensory ligaments hold the biconvex crystalline eye-lens that is just behind the pupil and iris. The muscle is responsible for changes in the shape of the lens for near and also far vision is termed as the ciliary muscle. The eye-lens is an elastic structure build up of protein fibres arranged similar to the layers of an onion. This is perfectly transparent and its focal length is around 3 cm.

The crystalline lens is followed through a dark chamber that is, is filled by vitreous humor. This is a transparent jelly-like substance. This augments the functions of aqueous humor and assists the eye hold its shape. The rear boundary of this chamber is the retina, where the image of the object is made. This comprises of a nervous layer and a pigmented layer. Apart from sensing the shape and the movement of an object, the retina as well senses its color. The retina comprises of five kinds of neuronal cells: the photoreceptors, bipolar, horizontal, amacrine and ganglion neurons.

The photoreceptor neurons are of two kinds: rods and cones. This is estimated that around 130 million rods and cones are found lining the retina. Of these, around 6 million are cones and approximately 20 times as many are rods.

The light sensitive pigments of photoreceptors are made up from vitamin A.

At the very centre of retina is a small yellowish depression, termed as fovea. This small valley (of around 5mm diameter) includes a large number (~110,000) of cones and no rods. The horizontal axis represents the distances in degrees of visual angle from the fovea positioned at 0°.

Rods are extremely specialized for vision in the dim light. They let us to distinguish between various shades of dark and light, see shapes and movements. That is, rods give a high sensitivity. Cones having light sensitive pigments that make color vision and sharpness of the vision (that is, high visual acuity) are possible.

If light is absorbed through photoreceptor cells, the light sensitive pigments are broken up through specific wavelengths of light. The bipolar nerve cells carry nerve impulses produced through rods and cones to the ganglion cells. The axons of the ganglion cells converge on the small area of the retina. This is lateral to the fovea and is free from rods and cones.

Image Formation:

Prior to stimulating rods and/or cones, light passes via the cornea, aqueous humor, pupil, eye-lens and vitreous humor. For clear vision, the image made on the retina must be sharp. Image formation on the retina comprises the refraction of light, accommodation of the eye-lens, constriction of the pupil and convergence of the eyes.

Refraction and Accommodation:

The light entering the eye via the transparent window - cornea experience refraction four times. This is due to the reason that the eye consists of four optically different media: cornea (n = 1.38), aqueous humor (n = 1.33), eye-lens (n = 1.40) and vitreous humor (n = 1.34). The maximum part of refraction takes place at the air-cornea interface. This is due to the reason that the cornea consists of a considerably bigger refractive index than air (n = 1.0). Furthermore, due to the curved shape, the cornea bends the light towards the retina. Additional bending is given by the eye-lens that is surrounded on both sides via eye-fluids. Through, the power of the lens to refract light is less than that of the cornea. Therefore the major function of the lens is to make fine adjustments in focusing. The focusing power of eye lens based on the tension in the ciliary muscle. If the ciliary muscle is relaxed, the lens is stretched and thinned. If a visual object is 6 m or more away from the eye, the cornea acquires nearly parallel light rays. If the eye is focusing an object closer than 6 m, the ciliary muscles contract. As an outcome, the lens shortens, thickens and bulges and its focusing power rises. The great value of the lens lies in its unique capability to automatically change its focal control. This capability is termed as accommodation. As accommodation means work for the muscles joined to the eye lens, viewing an object closer than 6 m for a long time can cause the eye-strain.

Constriction of the Pupil:

Constriction of the pupil signifies the narrowing down of the diameter of the hole via which light enters the eye. This action takes place concurrently by accommodation of the eye-lens and prevents the entry of light rays via the periphery of eye-lens that can result in blurred vision. The pupil as well constricts in bright light to protect the retina from sudden or intense stimulation. (If the level of illumination is low then the pupil dilates in such a way that the retina can get adequate light.)

Convergence:

Human beings encompass single binocular vision. This indicates that both eyes focus on just one set of objects. If we stare straight ahead at a distant object, the incoming light rays are directed at both pupils, get refracted and are focused at similar spots on the two retinas. Assume that we move close to the object and maintain our attention on the similar stationary object. Our general sense recommends that even now images must form on similar points (in both retinas). It in reality does occur and our eyes automatically make adjustments through radial movement of two eyeballs. This is termed to as convergence.

We will note that the images formed on the retina are inverted laterally and also up-side-down. However in reality we do not see a topsy-turvy world. The solution to this apparent question lies in the capacity of the brain that automatically processes visual images. This states that although vision starts in the eye, perception occur in our brain. Its proof lies in that severe brain injury can blind a person fully and permanently, even although the eyes carry on functioning perfectly.

Information Processing:

The instant light impulses impinge on the retina (and an image is made), these are absorbed through rods and cones that have four types of photosensitive substances. Such visual pigment molecules experience structural (that is, chemical) changes. This is supposed that each and every rod cell includes around seventy million molecules of a purple-colored photosensitive pigment, rhodopsin. Similar to rods, cones have violet - colored photosensitive pigment, iodopsin.

Each and every pigment molecule comprises of two components: a colorless protein, opsin and a colored chromophore, retinene. Opsin is dissimilar for each of the four visual pigments and finds out the frequency of light to which each and every pigment responds.

The above represents an understanding to what happens to rhodopsin in rods. (The similar fundamental changes take place in the visual pigments in cones.) The figure above portrays the rhodopsin cycle. The initial step in this process is the absorption of photon by rhodopsin, which then experiences a chemical change. Its cis-retinene part modifies to all-trans-retinene.

The rotation which takes place around the carbon numbered 12. This change triggers the decomposition of rhodopsin (into the scotopsin and all-trans-retinene) through a multi-phase process termed as bleaching action. The pigment loses color and the visual excitory event is believed to take place. Then rhodopsin is re-synthesized in the presence of vitamin A. In this method, an enzyme, retinene isomerase, plays the most fundamental role.

The rods respond even to poor illumination like twilight. Rhodopsin is very sensitive to even small amounts of light. Their responses to light produce colorless images and objects are observed only in shades of grey.

On the other hand, the pigments of the cones are less sensitive to the light and need bright illumination to start the decomposition of chromophore. Visual acuity or capability to see clearly and to differentiate two points close altogether is extremely high and their responses generate colored images.

The information acquired in terms of light is transformed into electrical signals in the retina. The potential of the cell membranes of the photoreceptor cells experiences a change even on brief illumination. This takes place via a complex chemical method comprising a flow of calcium ions and sodium ions across the membrane. The change in membrane potential, Δ V_m, is governed by the given equations in time and space:

Δ V_m (t) = I_m R (1 - e^-t/t)

Δ V_m (x) = V_o e^-x/L

Here, I_m is the membrane current, R is the membrane resistance and τ is the membrane time constant.  V_o is the change in the membrane potential at x = 0 (that is, x being the distance away from the site of current injection) and L is the length constant. As it is observed that, the spread of ΔV_m in space is governed through L (whose values fall in the range of around 0.1 to 1 mm). It is significant to note that as slow potentials are produced in most of the cells, action potentials are generated merely in the ganglion cells. The signals produced in the retina are further transmitted to the higher centers in the visual pathway of the brain like lateral geniculate nucleus and visual cortex. In this manner, accurate information regarding the image projected on the retina is conducted precisely to the brain.

Defects of Vision:

1) Short Sightedness (Myopia):

Meaning: This is the capability of a person to view near objects clearly however not distant objects. This eye defect is termed as short- sightedness.

Cause: This is mainly caused by the person's eye ball being too long and so focus far-off object in front of the retina. The far point for the eye is much closer than the normal eye.

Correction: A short sighted person must wear a concave (or diverging) spectacle lens. This spectacle lens (or concave) is utilized to form a virtual image of a distant object at person's far point.

Parallel rays from a far-away object appear to diverge from a far point (that is, the principal focus of the spectacle lens) and make them come out to come from a near distance. The short-sighted person has no problem of seeing near object closer to the eye than 25 cm as by the spectacle, the near point is slightly further away from eye.

2) Long-Sight (Hyper myopia):

Meaning: This is the ability of a person to see distant object clearly however not near object. General illustration is those who need glasses to read though not to see far-away object.

Cause: This is mainly caused by the person's eye ball being two short or eye lens being too thin or the optical system of the eye being too weak so that the rays from the normal near point (25 cm) are brought to focus beyond, rather than on, the retina. That is, rays from near objects focus behind the retina.

Correction: A convex (or converging) lens is employed for the correction of long-sight. The focal length must be such that it would generate a virtual image of an object placed 25 cm away at the person's near point.

Color Vision:

We are familiar that human beings have remarkable sense to adore the varied creations of nature. This is mostly because color is an automatic portion of our perception. However, the phenomenon of color vision has added real charm to life. The color is a perceptual experience; a formation of the eye and the mind.

One of the earliest examinations regarding color perception was made in the year 1825 by Purkinje. He noticed that at twilight, blue blossoms on flowers in his garden come out more brilliant than the red. The method of color perception is affected through the physiology of the eye and the psychology of the person. Prior to we plunge into these details, it is significant to know the dimensions of color, that is, the parameters by which color might be defined.

The Dimensions of Color:

The most significant physical dimension of color is the wavelength of light. For most of the light sources, what we recognize is the dominant color that we call the hue. This is hue to which we give the names like red, blue or greenish yellow. However, the terms color and hue are often employed interchangeably. You might thus conclude that hue is the perceptual correlate for variations in the wavelength.

The second dimension relevant to color vision is luminance that refers to the amount of light reaching the eye directly from the source. Illuminance, thus, characterizes the distinguished brightness of a colored light. This relationship (between illuminance and brightness) is quite complex as perceptual sensitivity differs by the wavelength of light. Each and every individual having normal eye have maximum sensitivity to light between the green and yellow portions of the spectrum (500nm to 600 nm). And the sensitivity to predominantly blue light (400 - 500 nm) is somewhat low.

The other physical dimension related with color is the degree of purity of spectral composition. That is, purity characterizes the extent to which the color (hue) appears to be mixed by white light. This is responsible for variations in the perceived saturations of the color. For illustration: if we add white light in a spectrally pure blue, the light starts to look sky-blue. On progressive addition of white light you might ultimately examine it as white.

Color Blindness:

A single monochromatic light can be generated through joining two primary colors. The measurements made to recognize the amounts of these colors needed to match a given monochromatic color gave fairly standard results. That is, if we ask a group of people to match a test color, experience sates that they mix the similar proportions of primary colors. However color-mixing requirements for some individual might be anomalous. However, a few individuals might require only two, instead of three, primary colors to match all the monochromatic hues. Such anomalies are indicative of varying degrees of color blindness. People who represent anomalous color-matching requirement don't observe the similar colors as individuals having normal vision. The most general defect is in the proportions of green and red lights needed to match a monochromatic yellow. The manifestation of this in daily life is a limited capability to differentiate between red and green.

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