Human Eye

THE HUMAN EYE AND THE COLOURFUL WORLD Anchli Singh, Class XA

The Human Eye The human eye is a significant human sense organ. It allows humans conscious light perception, vision, which includes color differentiation and the perception of depth. The human eye has a 200° viewing angle and can see 10 million colors

The Human Eye

Human eye is like a camera Its lens system forms an image on the light sensitive screen called the retina. Light enters the eye through a thin membrane called the Cornea. The eyeball is approximately spherical in shape with a diameter of 2.3 cm. Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea. The crystalline lens merely provides the finer adjustment of focal length required to focus objects at different distances on the retina. We find the structure called the Iris behind the cornea. Iris is a dark muscular diaphragm that controls the size of the pupil. The pupil regulates and controls the amount of light entering the eye. The eye lens forms an inverted real image of the object on the retina. The Retina is a delicate membrane having enormous number of light sensitive cells. The light-sensitive cells get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals ad finally processes the information so that we perceive objects as they are.

Power of accommodation •

Accommodation of the Eye The process by which the ciliary muscles change the focal length of an eye lens to focus distant or near objects clearly on the retina is called the accommodation of the eye.

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Power of Accommodation The ability of the eye to focus objects lying at different distances is called the power of accommodation of the eye.

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How Does an Eye Focus Objects at Varying Distances? To focus on distant objects the ciliary muscles relax making the eye lens thin. As a result the focal length of the eye lens increases and we see the distant objects. But to focus on nearby objects the ciliary muscles contract making the eye lens thick. As a result the focal length of the eye lens decreases and we see the nearby objects. In short it is the adjustment of the focal length of the eye lens which enables us to focus on objects situated at different distances.

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Near point or Least Distance of Distinct Vision Near point or least distance of distinct vision is the point nearest to the eye at which an object is visible distinctly. For a normal eye the least distance of distinct vision is about 25 centimeters. However, it varies with age of the person. For example, for infants it is only 5 to 8 cm. Far Point Far point of the eye is the maximum distance up to which the normal eye can see things clearly. It is infinity for a normal eye.

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Range of Vision The distance between the near point and the far point is called the range of vision.

Defects of vision and correction MYOPIA •

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You must have seen some people holding books very close to their eyes. This is because they suffering from myopia (short sightedness). A myopic person cannot see distant objects clearly because the far point of his eye is less than infinity. Myopia is an eye defect due to which the eye is not able to see distant objects clearly. This occurs when the light rays entering the eye converge in front of the retina in the vitreous body. Here you can see the formation of image in a normal eye and in a myopic eye. In the case of a normal eye the rays of light from the object fall on the eye and converge on the retina but in the case of a myopic eye the light rays are focused in front of the retina.

Defects of vision and correction MYOPIA Myopia is the defect of the eye due to which the eye is not able to see the distant objects clearly.

CAUSES * the elongation of the eye ball, that is, the distance between the retina and eye lens is increased * decrease in focal length of the eye lens A myopic eye forms the image of a far off object in front of the retina because of the increase in converging power of the eye lens. Therefore myopia can be rectified by using a suitable divergent or concave lens. The ray diagram shows how a concave lens helps in focusing the light on the retina. This defect is commonly known as nearsightedness.

Defects of vision and correction HYPERMETROPIA •

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You must have seen middle aged people holding a book away from their eyes to read properly. This is because they are not able to see the nearby objects clearly. We say that those people are suffering from Hypermetropia (long sightedness). Hypermetropia or Hyperopia is an eye defect in which distant vision is clear while near vision is blurred. This occurs when the light rays entering the eye converge behind the retina. You can see the formation of image in a normal eye and in a Hypermetropic eye. In the case of a normal eye the rays of light from the object fall on the eye and converge on the retina but in the case of a hypermetropic eye the light rays are focused behind the retina.

Defects of vision and correction HYPERMETROPIA Hypermetropia is the defect of the eye due to which the eye is not able to see the near objects clearly.

CAUSES • •

Shortening of the eyeball, that is, the eyeball becomes smaller Increase in focal length of the eye lens Let us now see how this defect is rectified. A long sighted eye forms image of a nearby object behind the retina. Thus, long sightedness is due to the decreased converging power of the lens. Therefore Hypermetropia can be rectified by making the eye lens more convergent. This is done by placing a convex lens of suitable focal length before the eye lens as shown in the figure.

Defects of vision and correction PRESBYOPIA The power of accommodation of the eye usually decreases with ageing. For most people, the near point gradually recedes away. They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is called Presbyopia. It arises due to gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. Sometimes, a person may suffer from both Myopia and Hypermetropia. Such people often require biconcave and convex lenses. The upper portion consists of a concave lens for distant vision and the lower part consists of a convex lens that facilitates near vision.

Dispersion of White Light By a Glass Prism Even though all colours of the visible spectrum travel with the same speed in vacuum, the speed of the colours of the visible spectrum varies when they pass through a transparent medium like glass and water. That is, the refractive index of glass is different for different colours. When a polychromatic light (multi coloured or light containing more than one wavelength) like white light is incident on the first surface of the prism it gets refracted. But each constituent of the white light gets refracted through a different angle, i.e., white light gets dispersed. When these colours are incident on the second surface of the prism they again undergo refraction (they get refracted from a denser to rarer medium) and the colours are separated further. Thus a beam of white light incident on a prism splits into its constituent colours to form a spectrum. Each constituent of the white light is deviated towards the base of the prism. Violet colour suffers the maximum deviation and red the least. The spectrum obtained is impure as the colours in the spectrum do not have any sharp boundaries i.e., each colour merges gradually into the next.

ATMOSPHERIC REFRACTION Atmospheric refraction is the shift in apparent direction of a celestial object caused by the refraction of light rays as they pass through Earth’s atmosphere. TWINKLING OF STARS The twinkling of stars and variation in size of the Sun are due to atmospheric refraction of starlight. Do stars really twinkle? No, stars do not twinkle. Let us now learn about why they appear to do so. The rays of light coming from the stars travel through the layers of air of varying densities. These rays get refracted continuously and they bend towards the normal as the refraction is from a rarer to a denser medium. The movements of air and convection currents cause a change in the density of the layers of air. As a result, the position of the image of the star goes on changing after every short interval. These different positions of the images formed at short intervals of time give the impression that the star is twinkling.

ATMOSPHERIC REFRACTION ADVANCE SUNRISE AND DELAYED SUNSET At dusk or dawn the Sun appears to be larger than at noon. This is because when the sun is near the horizon the rays of light coming from the sun have to pass through layers of air of increasing density. Due to continuous bending of light the sun appears to be larger. At noon, the sun appears to be smaller than at dusk or dawn. This is because the rays of light that fall normally on the surface of the earth do not get refracted.

SCATTERING OF LIGHT Scattering is a general physical process whereby some forms of radiation, such as light or moving particles, for example, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which it passes. A large number of molecules are present in the earth’s atmosphere. These molecules scatter light in various directions. The air is composed of many tiny particles including dust and water vapour. As the sunlight passes through the air, the shorter blue light waves are reflected and refracted by the particles while the other coloured light waves being longer are unaffected and are not reflected by the water vapour or dust in the air. Blue, therefore, is scattered the most and this explains the bluish colour of the sky. At sunset or sunrise, the sunrays have to cover large atmospheric distances to reach us and most of the blue light gets scattered and doesn’t reach us. The sky as well as the sun, at sunrise and sunset, therefore looks reddish.

SCATTERING OF LIGHT TYNDALL EFFECT The earth’s atmosphere is a heterogeneous mixture of minute particles. These particles include smoke, tiny water droplets, suspended particles of dust and molecules of air. When beam of light strikes such air particles, the path of the beam becomes visible. Similarly the path of a beam of light passing through a true solution is not visible. However, its path becomes visible through a colloidal solution where the size of the particles is relatively larger. The phenomenon of scattering of light by the colloidal particles gives rise to Tyndall effect. Tyndall effect is the visible scattering effect of light on particles along the path of a beam of light passing through a colloid system.

What have we learnt? • •

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The ability of the eye to focus both near and distant objects, by adjusting its focal length is called the accommodation of the eye. The smallest distance at which we can see objects clearly is called the Near Point of the eye or the least distance of distinct vision. For a young adult the normal vision is about 25 cm. The common refractive defects of vision include myopia, hypermetropia and presbyopia. Myopia- short sightedness, is corrected by using a concave lens. Hypermetropia-far sightedness, is corrected by using a convex lens. Presbyopia uses both types of lens for correction. The splitting of white light into its components and colours is called Dispersion. Scattering of light causes blue colour of the sky and the reddening of the sun at sunrise and sunset.

Thank you all for your kind attention and patient listening!!!