Human Eye and The Colourful World PDF

Human Eye and The Colourful World The Human Eye The human eye is one of the valuable and sensitive sense organs in the human body like camera. It enables us to see the wonderful world and colours around us. The human eye is like a camera, its lens system forms an imag...

Human Eye and The Colourful World The Human Eye The human eye is one of the valuable and sensitive sense organs in the human body like camera. It enables us to see the wonderful world and colours around us. The human eye is like a camera, its lens system forms an image on a light- sensitive screen called retina. Structure of Human Eye The human eye has the following main parts:  Cornea It is the transparent bulged out spherical membrane covering the front of the eye. Light enters the eye through this membrane. Most of the refraction of light rays entering the eye occurs at the outer surface of the cornea.  Crystalline lens The eye lens is a convex lens made of a transparent, soft and flexible jelly-like material made of proteins. The eye-lens is held in position by suspensory ligaments.  Iris It is a dark muscular diaphragm between the cornea and the lens. It controls the size of the pupil. It is the colour of the iris that we call as the colour of the eye.  Pupil It is a small hole between the iris through which light enters the eye. In dim light, it opens up completely due to expansion of eye muscles, but in bright light it becomes very small due to contraction of eye muscles.  Ciliary Muscles They hold the lens in position and help in modifying the curvature of the eye lens.  Retina It is the light-sensitive surface of the eye on which the image is formed. It contains light- sensitive cells known as rods and cones. Rod cells response to the illumination. i.e. primary colours. Number of rod cells is greater than number of cone cells. These cells generate signals which are transmitted to the brain through optic nerves.  Optic nerve It transmits visual information from the retina to the brain.  Sclera It is an opaque, fibrous, protective, outer layer of an eye containing collagen and elastic fibre. It is also known as white of the eye.  Blind spot It is the point at which the optic nerve leaves the eye. It contains no rods and cones, so an image formed at this point is not sent to the brain.  Aqueous Humour Between the cornea and eye lens, we have a space filled with a transparent liquid called the  aqueous humour which help the refracted light to be focused on retina. It also maintains intraocular pressure.  Vitreous Humour The space between the eye lens and retina is filled with another liquid known as vitreous humour Note * Iris regulates the amount of light entering the eye by adjusting the size of the pupil. * The pupil appears black because no light is reflected by it. * The eye ball is nearly spherical in shape with a diameter of 2.3 cm. * If cornea, Pupil, eye lens, retina, optic nerves, aqueous humour and vitreous humor malfunction or get damaged it will result in visual impairment. Formation of an image An image is formed on the retina by successive refractions at the cornea, the aqueous humour, lens and vitreous humour. It is real, diminished and inverted in nature. The light-sensitive cells of retina get activated upon illumination and generate electrical signals. These signals are then sent to the brain via the optic nerve. The brain interprets these signals and finally processes the information’s, so that we perceive objects as they are. Colour of Objects The rod shaped cells of retina respond to the intensity of light, i.e. the degree of brightness and darkness but do not respond to colours. The cone cells are sensitive to the different extent of primary colours such as red, blue and green. Note: Eyesight of Bees The retinal cones of bees are sensitive to the ultraviolet light which we cannot see with our eyes. Eyesight of Chicks - Chicks wake up earlier in the morning than humans because their retina have mostly cones which are very sensitive to bright light and rods are very few. Terms Related to Human Eye (i) Accommodation It is the ability or the property of the eye lens to focus both near and distance objects by adjusting its focal length. However, the focal length cannot be decreased or increased beyond a certain limit, due to which a healthy person cannot view clearly, if the object is held too close (i.e. less than 25 cm) or too far from the eye. Power of accommodation of eye The ciliary muscles help in changing the curvature of the eye lens. When muscles are relaxed, the lens becomes thin and its focal length is increased. This enables us to see distant objects clearly. While viewing nearby objects, the ciliary muscles contract. So, the lens becomes thicker and its focal length decreases. (ii) Power of accommodation It is the maximum variation in power of eye lens for focusing near by or far objects, clearly at retina. For a young adult with normal vision, the power of accommodation is about 4D. the eye loses its power of accommodation at old age. (iii) Far point of the eye It is the farthest point up to which the eye can see clearly. It is infinity for normal eye. (iv) Near point of the eye The minimum distances, at which an object can be seen most distinctly without any strain is called the least distance of distinct vision. For a normal eye of an adult, it is 25 cm. It is also called near point of the eye.  (v) Persistence of vision The time for which impression or sensation of an object continues to remain in the eye is called persistence of vision. It is about 1/16 th of a second which means that the minimum time for which we should view an object, so that its clear image is formed on retina is 1/16 th of a second. Note: When the eye is looking at the nearby objects, eye lens becomes more convex (i.e. focal length decreases) Defects of Vision and their Correction The defects due to which a person cannot see the objects distinctly and comfortably are called defects of vision. The main defects of vision are: (i) Myopia or near / short sightedness (ii) Hypermetropia or far / long sightedness (iii) Presbyopia Myopia or near / short sightedness In this defect, a person can see nearby objects distinctly but cannot see distant objects clearly. In this case, image is formed before retina and not on the retina. Causes A person with this defect has a far point nearer than infinity. This defect due to the decrease in focal length of the lens because of  Excessive curvature of eye lens,  Elongation of the eyeball. (due to eye ball being too long) As a result, the image is formed before retina. Remedy This defect can be corrected by using concave lens. A concave lens of suitable power will bring back the image on retina. a) Far point of myopic eye b) Myopic eye c) Correction for myopic eye Hypermetropia or far / long sightedness In this defect, a person can see distant objects clearly but cannot see nearby objects clearly. A person with this defect has the near point farther away from normal near point (25 cm). in this case, the image is formed beyond retina. Causes This defect arises due to following reasons:  Focal length of eye lens becomes large,  Eyeball becomes too short, so that the image is formed behind retina. Remedy This defect can be corrected by using a convex lens of suitable power. This will bring the image back on retina. a) Near point of hypermetropic eye b) Hypermetropic eye c) Correction for hypermetropic eye Presbyopia It is found in old people. For most of the people, the near point gradually recedes away with age. Sometimes, a person may suffer from both myopia and hypermetropia. Causes This defect arises due to the following reasons:  Weakness of ciliary muscles.  Hardening or loss of elasticity of eye lens. Remedy This defect can be corrected by using bifocal or varifocal lenses which consist of both convex and concave lenses. The upper portion consists of a concave lens (for myopia) and lower portion consists of convex lens (for hypermetropia). Numerical problems There are many numerical problems which are generally based on myopia and hypermetropia on finding the focal length and power of the lens which are used to correct the defect. These problems are solved by using lens formula. = Where, f = focal length of the lens (if f is + ve, then lens is convex and if f is – ve, then lens is concave lens.) V = image distance and u = object distance. In these problems, always remember that for healthy human eye, the far point is infinity and near point is least distance of distinct vision, i.e. 25 cm. Refraction of Light through a Prism Prism is a transparent refracting medium bounded by at least two lateral surfaces, inclined to each other at a certain angle. It has two triangular bases and three rectangular lateral surfaces. The angle between two lateral surfaces is called angle of prism (A). In the diagram given above, a ray of light PQ is entering from air to glass at the first surface AB. The light ray on refraction is bent towards the normal. At the second surface AC, the light ray enters from glass to air, so it bents away from the normal. The above diagram shows refraction through a prism, Where PQ = incident ray, QM = refracted ray, MR = emergent ray, A = angle of prism, = angle of incidence, r = angle of refraction, e = angle of emergence, D = angle of deviation Angle of deviation (D) It is the angle at which the emergent ray (extended backward) makes with the incident ray (extended forward). It depends upon angle of prism, i.e. ( A), angle of incident ( ) and angle of emergence ( e) and is given by D= + e A Dispersion of white light by a Glass Prism The phenomenon of slitting of white light into its constituent colours, when it passes through a prism is called dispersion. This band of seven colours so obtained, the VIBGYOR (V = violet, I = indigo, B = blue, G = green, Y = Yellow, O = orange and R = Red) is called spectrum. Isaac Newton was the first one to use a glass prism to obtain the spectrum of light. Cause of Dispersion Light rays of different colours, travel with the same speed in vacuum and air but in any other medium, they travel with different speeds and bend through different angles, which leads to the dispersion of light. Red light has the maximum wavelength and violet light has the minimum wavelength. So in any medium, red light travels fastest and deviates least, while violet light travels slowest and deviates maximum, i.e. Wavelength Recombination of White Light Newton showed that the reverse of dispersion of light is also possible. he kept two prisms close to each other, one in erect position and the other in an inverter position. The light gets dispersed when passes through the first prism. The second prism receives all the seven coloured rays from first prism and recombines them into original white light. This observation shows that sunlight is made up of seven colours. Any light that gives spectrum similar to that of sunlight is called white light. Rainbow A rainbow is a natural spectrum appearing in the sky after a rain shower. It is caused by dispersion of sunlight by tiny water droplets, present in the atmosphere. A rainbow is always formed in a direction opposite to that of the Sun. the water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally and finally, reflect it again when it comes out of the raindrop. Due to the dispersion of light and internal reflection, different colours reach the observer’s eye. A rainbow can also be seen on a sunny day by looking at the sky through a waterfall or through a water fountain, with the Sun behind you. Atmospheric Refraction The Earth’s atmosphere is not uniform throughout; its density goes on changing as we move up or down. It can be considered to be consisting of layers of different densities, which act as rarer or denser medium with respect to each other. Due to this, when the light rays pass through the earth’s atmosphere, they undergo refraction. The refraction of light caused by these layers is called atmospheric refraction. Some Phenomena Based on Atmospheric Refraction. Twinkling of Stars The twinkling of a star is due to atmospheric refraction of starlight. As the light from the star enters the earth’s atmosphere, it undergoes refraction due to varying optical densities of air at various altitudes. The continuously changing atmosphere refracts the light by different amounts. In this way, the starlight reaching our eyes increases and decreases continuously and the star appears to twinkle at night. The star seem higher than they Actually Are As the light from a star enters the Erath’s atmosphere, it undergoes refraction and bends towards the normal each time due to the atmospheric refraction. Therefore, the apparent position of the star is slightly different from its actual position. The star appears to be slightly higher than its actual position, when viewed near the horizon. Planets do not Twinkle As planets are of larger size and much closer to the earth, than stars they can be considered as a collection of large number of point sized sources of light. The total variation in the amount of light entering our eye from all these individual point sized sources will average out to zero which nullify the twinkling effect of each other. Therefore, planets do not twinkle. Advance Sunrise and Delayed Sunset The sun is visible to us about two minutes before the actual sunrise and about two minutes after the actual sunset. This is because of atmospheric refraction. When the Sun is slightly below the horizon, the sunlight coming from the less dense to more dense air, is refracted downwards. Because of this, the Sun appears to be raised above the horizon and so the Sun can be seen about two minutes before actual sunrise. Similarly, due to atmospheric refraction, the Sun can be seen for about two minutes even after the Sun has set below horizon. Scattering of Light The refraction of light from an object in all directions is called scattering of light. The colour of scattered light depends on the size of scattering particles and wavelength of light. Very fine particles scatter mainly blue light while particles of larger size scatter light of longer wavelength (red light). If the size of the scattering particles is large enough, then the scattered light may even appear white. Some Phenomena Based on Scattering of Light Tyndall Effect A beam of light passing through a true solution is not scattered. The scattering of light when it passes through a colloidal solution is called Tyndall effect. The earth’s atmosphere is a heterogeneous mixture of minute particles of smoke, tiny water droplets, suspended particles of dust and molecules of air which becomes visible due to scattering light. Why the Colour of the Sky is Blue? During the day time, sky appears blue. This is because the size of the particles in the atmosphere is smaller than the wavelength of visible light, so they scatter the light of shorter wavelengths. The scattered blue light enters our eye. It should be noted that the sky appears black to the passengers flying at higher altitudes because scattering of light is not prominent at such height due to the absence of particles. Colour of Sun at Sunrise and Sunset At sunrise and sunset, the Sun and the Sky appear red. Light from the Sun near the horizon passes through thicker layers of air and covers larger distance in the atmosphere before reaching our eyes. Near the horizon, most of the blue light and shorter wavelengths light rays are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelengths. This gives rise to the rediish appearance of the Sun and the Sky. However at the noon, the light from the Sun overhead would travel relatively shorter distance. So, it appears white as only a little of the blue and violet colours are scattered.  It is a pyramidal piece of glass with two triangular bases and three rectangular faces. Dispersion of White Light by a Glass Prism – - The phenomenon of splitting of white light into its 7 colors, when it passes through a prism is called Dispersion. - This band of seven colors VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, red) is called Spectrum Ques) Why Dispersion Occurs? Ans - Light rays of different colors, travel with the same speed in vacuum and air but in any other medium, they travel with different speeds and bend through different angles, which leads to dispersion of light.  Recombination of White Light - Reverse of dispersion is also possible - Seven Colored lights of the spectrum can be recombined to give back white light by placing two prisms, one upside down. Total Internal Reflection When light enters obliquely from a denser medium to a rarer medium and the angle of incidence exceeds critical angle, the light reflects in the denser medium. This is called Total Internal Reflection. Conditions necessary for Internal Reflection i) Light should enter obliquely from a denser to a rarer medium. ii) The angle of incidence should exceed critical angle, the light reflects in the denser medium. Critical Angle – The angle of incidence for which the angle of refraction is 90o A natural spectrum appearing in sky after rain. - Some Water droplets remain in air after Rain. These droplets act as small prisms. - Water droplets refract and disperse the incident sunlight, then reflect internally and finally, reflect it again when it comes out of raindrop. - Rainbow is always in opposite side of Sun. Refraction Light Reflection The refraction by different layers of atmosphere is called Atmospheric Refraction. Effects of Atmospheric Refraction – 1) Advanced Sunrise - The sun appears about two minutes earlier than actual sunrise and the sun remains visible for about two minutes after actual sunset. - When the sun is below horizon, the rays have to pass from rarer to denser medium. So rays bend towards the normal. As a result the sun appears higher than its actual position. Perceived Position True Position  2) An object placed behind the fire appears to flicker - The air above hot surface becomes hot and rises. The space is occupied by cool air. The refractive index of hot air is less than that of cool air. So the physical condition of the medium are not constant. Due to changing Refractive Index of medium, the light appears to come from different directions. It results in fluctuations in apparent position of object. Flickering 3) Star seen higher than they actually are - The refractive index of earth’s atmosphere in general increases from top to bottom. So, the light coming from a star near the horizon has to travel from rarer to denser medium and it bends towards the normal. Due to which stars appears higher.  Star seems higher Actual position of Star Increasing Bending of Refractive Index Star’s light due to atmospheric refraction 4) Twinkling of Stars - Stars are very far from us, so they behave as point source of light. Since the physical conditions of the earth’s atmosphere are not constant the light from stars appears to come from different directions. This results in fluctuation of apparent position of star. - The amount of light coming from stars also vary due to changing Refractive Index of atmosphere. - The stars appears bright when more light from star reaches our eyes and the same star appears dull when less amount of light reaches our eyes. This causes twinkling of Stars. Ques) Why do planets not twinkle? Ans - The planets are much closer to the earth and are considered a collection of many point sized sources of light, the total amount of light entering our eye from all the individual point sized sources will average out to zero, thereby nullifying the twinkling effect. The reflection of light from an object in all directions is called scattering of light. - It depends on the size of particle ( Particle Scatter ई Light) i) Small size particles scatter blue color of light (Shorter wavelength) ii) Medium size particles scatter red color of light (Longer wavelength) iii) Large size particles scatter all the colors of light that’s why it appears white 1 Scattering ∝ wavelength 1) Tyndall Effect - The earth’s atmosphere is a heterogeneous mixture of minute particles of smoke, tiny water droplets, dust in air which becomes visible due to scattering of light. Tyndall Effect 2) Color of Sky is Blue - The upper layer of atmosphere contains very small particles of water vapours and gases. These particles are more effective in scattering of light of shorter wavelength mainly blue color, So sky appears blue. - If earth had no atmosphere than there would not have been any scattering of light because at higher altitude there are no particles that’s why sky of other planets is dark because there is no atmosphere in planets like Mars. - So sky appears dark to an Astronaut in space or to a passenger of jet plane 3) Danger Signs are of Red Color - Red is the least scattered color. It is least scattered by fog and smoke can be seen in the same color over a long distance. So, danger signs are made in red color. 4) Color of Sun at Sunrise and Sunset - During sunset and sunrise, the sun is near horizon and therefore the sunlight has to travel larger distance in atmosphere. Due to this most of the blue light (shorter wavelength) are scattered away by the particles. The light of longer wavelength (red color) will reach our eye. This is why sun appear red in color