Human eye class 10


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Human eye class 10

  1. 1. WISDOM ACADEMY 1 ARUNACHAL,KALI BARI BAZAR HUMAN EYE AND COLOURFUL WORLDHuman EyeEye is one of the most sensitive sense organs in the human body. Our eye enables us to see thisbeautiful world. It consists of a lens, which is made up of living tissues.Structure of human eyeThe human eye is roughly spherical inshape with diameter of about 2.3 cm. It consists of a convex lens made up of living tissues. Hence, human lensesare living organs contrary to the simpleoptical lenses. The following table lists the main parts of the human eye andtheir respective functions.The white of the eye is known as the sclera. It is the tough, opaque tissue that protects the outerlayer of the eye. Iris is the coloured part of the eye, and the pupil is the black, Circular hole that islocated at the centre of the iris. The thin, transparent tissue that covers the outer surface of the eye isknown as the conjunctiva. It consists of tiny blood vessels that nourish it.1. Pupil: Pupil is the round black spot in front of eye. It regulates the amo unt of light entering the eyes.Pupil works like aperture of a camera. In case of dim light pupil dilate to allow more light to enter the eyes.In case of strong light pupil constrict allowing less light to enter.2. Irish: Irish is made of muscles. They c ontrol the size of opening if pupil.3. Lens: Lens lies just behind the pupil. Lens becomes thin to increase its focal length. This enables us to seedistant objects clearly. To focus on nearer objects lens becomes thick to decrease its focal length. But there is alimit. The minimum distance of clear vision is 25 cm. Below this distance we cannot see things clearly.4. Retina: Retina works like a screen or camera film. Reti na is full of light and colour sensitive cells.These cells, upon receiving image send electrical signals to the brain, which processes these informationto make a mental image of what we see.5. Cornea : The cornea is located at the front portion of the eye. It is the transparent window that bulgesoutwards. The lens consists of layers of tissues enclosed in a tough capsule. The focus of the lens isadjusted by the ciliary muscles that suspend and hold it.SANDIP CHAKRABORTY. M: 9577556400,9435899571 Page 1
  2. 2. WISDOM ACADEMY 2 ARUNACHAL,KALI BARI BAZARFunction of the human eyeLight rays enter the eye through the cornea. The rays are bent, refracted, and focused by the cornea,lens, and the vitreous humour. The main function of the lens is to focus the light rays sharply on theretina. It is the outer surface of the cornea where most of the refraction of light occurs.Iris and pupil control the size of the pupil and the amount of light respectively. Since the eye lensis convex in nature, the resulting image is real, small, and inverted. This image is formed on theretina.The retina converts these light rays into electrical signals with the help of light sensitive cells. Thesesignals are sent to the brain via translated and perceived objects in an erect or upright positionThe head of the optic nerve is devoid of photosensitive cells. Hence, no image is formed at that pointcalled the blind spot of the eyeOn sunny days, when you enter a dimly lit room, you are unable to see clearly for a moment.Why does this happen?In bright light, the iris expands, thereby contracting the pupil. This happens so that only a small quantityof light enters the eye. As a result, the retina is protected from exposure to excessive lightOn entering a dimly lit room after having been in the sun for some time, the iris contracts slowly toexpand the pupil. Gradually, more light is able to enter the eye. Hence, it takes a few seconds before weare able to see the objects present in the dimly lit room.Q. What happens to the image distance in the eye when we increase the distance of an objectfrom the eye?Ans. The image is formed on the retina even on increasing the distance of an object from the eye.For this eye lens becomes thinner and its focal length increases as the object is moved away fromthe eyeColour perception Have you wondered how do we see colour?Retina consists of two types of light sensitive cells – rod cells and cone cells. The rod shaped cells respondto the intensity or brightness of the focussed light whereas the cone shaped cells of the retina respond tothe colours. Thus, the cone cells of the retina make colour perception possibleA person having defective cone cells is not able to distinguish between the different colours. This defect isknown as Colour Blindness.Benefits of two eyes: One eye is having a field of vision of about 150 degrees. Both theeyes enable us to see up to a field of 180 degrees. Moreover, as two differentimagesget juxtaposed in the brain, so we are able to see a three dimensional vi ew ofthe world.Have you ever thought why animals’ eyes are positioned on their heads?This is because it provides them with the widest possible field of view. Our eyes are locatedin front of our face. One eye provides 150° wide field of view while both eyes simultaneouslyprovide 180° wide field of view. It is the importance of the presence of two eyes as both eyestogether provide the three- dimensional depth in the image.Have you wondered why the eye is able to focus the images of objects lying at variousdistancesIt is made possible because the focal length of the human lens can change i.e., increase ordecrease, depending on the distance of objects. It is the ciliary muscles that can modify thecurvature of the lens to change its focal lengthSANDIP CHAKRABORTY. M: 9577556400,9435899571 Page 2
  3. 3. WISDOM ACADEMY 3 ARUNACHAL,KALI BARI BAZARTo see a distant object clearly, the focal length of the lens should be larger. For this, the ciliarymuscles relax to decrease the curvature and thereby increase the focal length of the lens. Hence,the lens becomes thin. This enables you to see the distant object clearly.To see the nearby objects clearly, thefocal length of the lens should beshorter. For this, the ciliary musclescontract to increase the curvature andthereby decrease the focal length ofthe lens. Hence, the lens becomes thick. This enables you to seethe nearby objects clearly.Power of accommodationThe ability of the eye lens to adjust its focal length accordingly as the object distances is called powerof accommodation.The minimum distance of the object by which clear distinct image can be obtained on the retina is calledleast distance of distinct vision. It is equal to 25 cm for a normal eye. The focal length ofthe eye lens cannot be decreased below this minimum limit of object distance.The far point of a normal eye is infinity. It is the farthest point up to which the eye can see objects clearly.The range of vision of a normal eye is from 25 cm to infinity.Q. Why is a normal eye not able to see clearly the objects placed closer than 25 cm ? Ans. Due to a power of accommodation, the focal length of the eye lens cannot be decreasedbelow a certain minimum limit. Consequently, a normal eye cannot see clearly the objects placed closerthan a minimum distance, called near point of eye.For a young adult value of near point is 25 cm. So, we cannot see clearly objects placed closer than 25cm.Q . What is the far point and near point of the human eye with normal vision?Ans. For a human eye with normal vision the far point is at infinity and the near point is at 25 cm fromthe eye.Q . The human eye can focus objects at different distance by adjusting the focal length of theeye lens. This is due to (a) presbyopia (b) accommodation (c) near-sightedness(d) far-sightednessAns. (b) accommodationQ . The human eye forms the image of an object at its (a) cornea (b) iris (c) pupil (d) retinaAns. (d) retina.Q . The least distance of distinct vision for a young adult with normal vision is about(a) 25 m (b) 2.5 cm (c) 25 cm (d) 2.5 m Ans. (c) 25 cm.Q . The change in focal length of an eye lens is caused by the action of the(a) pupil (b) retina (c) ciliary muscles. (d)irisAns. (c) ciliary muscles.SANDIP CHAKRABORTY. M: 9577556400,9435899571 Page 3
  4. 4. WISDOM ACADEMY 4 ARUNACHAL,KALI BARI BAZARDefects of vision.The loss of power of accommodation of an eye results in the defects of vision.There are three defects of vision called refractive defects. They are (i) myopia, (ii) hypermetropia,and (iii) presbyopia. In this section, we will learn about these defects of vision in detail.1. Myopia (short sightedness)Myopia is a defect of vision in which a person clearly sees all the nearby objects, but is unable to see thedistant objects comfortably and his eye is known as a myopic eye. A myopic eye has its farpoint nearer than infinity. It forms the image of a distant object in front of its retina as shown inthe figureMyopia is caused byi. increase in curvature of the lens (ii) increase in length of the eyeballSince a concave lens has an ability to diverge incoming rays, it is used to correct this defect ofvision. The image is allowed to form at the retina by using a concave lens of suitable power as shownin the given figure.A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What shouldbe the type of the corrective lens used to restore proper vision?Ans. A person with a myopic eye should use a concave lens of focal length 1.2 m so as to restoreproper vision.Q . A student has difficulty reading the blackboard while sitting in the last row. What could bethe defect the child is suffering from? How can it be corrected?Ans. The student is suffering from myopia or short-sightedness. The defect can be correctedby the use of concave (diverging ) lens of an appropriate power.SANDIP CHAKRABORTY. M: 9577556400,9435899571 Page 4
  5. 5. WISDOM ACADEMY 5 ARUNACHAL,KALI BARI BAZAR2. Hypermetropia (Long sightedness)Hypermetropia is a defect of vision in which a person can see distant objects clearly anddistinctively, but is not able to see nearby objects comfortably and clearly The problem with ahypermetropic eye can be solved with the help of a diagram. It is shown in thegiven figure.A hypermetropic eye has its least distance of distinct vision greater than 25 cm.Hypermetropia is caused due to i. reduction in the curvature of the lens ii. decrease in the length of the eyeballSince a convex lens has the ability to converge incoming rays, it can be used to correctthis defect of vision, as you already have seen in the animation. The ray diagram for thecorrective measure for a hypermetropic eye is shown in the given figureSANDIP CHAKRABORTY. M: 9577556400,9435899571 Page 5
  6. 6. WISDOM ACADEMY 6 ARUNACHAL,KALI BARI BAZAR3. Presbyopia (Ageing vision defect)Presbyopia is a common defect of vision, which generally occurs at old age. A person sufferingfrom this type of defect of vision cannot see nearby objects clearly and distinctively. Apresbyopic eye hasits near point greater than 25 cm and it gradually increases as the eye becomes olderPresbyopia is caused by the(i) weakening of the ciliary muscles(ii)reduction in the flexibility of the eye lensA person with presbyopia cannot read letters without spectacles. It may also happenthat a person suffers from both myopia and hypermetropia. This type of defect can be correctedby using bi-focal lenses. A bifocal lens consists of both convex lens (to correcthypermetropia) and concave lens (to correct myopia).It is a common misconception among people that the use of spectacles “cures” the defects ofvision. However, this is not true as spectacles only “restore” the defects of vision to thenormal value.CataractIt is also one of the eye defects found commonly in people of older ages. In this defect, thecrystalline lens becomes milky and cloudy. This condition is also known as cataract. Thiscauses partial or complete loss of vision. This loss of vision can be restored by removing thecataract by means of a cataract surgery. The use of any kind of spectacle lenses does notprovide any help against this defect of vision.Refraction Of Light Through A Glass Prism And Dispersion Of WhiteLightWhen a ray of light is incident on a rectangular glass slab, after refracting through the slab, itgets displaced laterally. As a result, the emergent ray comes out parallel to the incident ray.Does the same happen if a ray of light passes through a glass prism?Unlike a rectangular slab, the sides of a glass prism are inclined at an angle called the angle ofprism. Therefore, a ray of light incident on its surface, after refraction, will not emerge parallelto the incident light ray (as seen in the case of a rectangular slab).Refraction of light through a glass prismSANDIP CHAKRABORTY. M: 9577556400,9435899571 Page 6
  8. 8. WISDOM ACADEMY 8 ARUNACHAL,KALI BARI BAZARTake a glass prism and allow a narrow beam of sunlight to fall on one of its rectangular surfaces. You will obtaina coloured spectrum with red and violet colour at its extreme. Try to obtain a sharp coloured band on the screenby slightly rotating the prism. Count the colours of the band and write the sequence of the colours.When a beam of white light AB enters a prism, it gets refractedat point B and splits into its seven constituent colours, viz. violet, indigo, blue, green, yellow, orange, and red. The acronym for the seven constituent colours of white light isVIBGYOR. This splitting of the light rays occurs because ofthe different angles of bending for each colour. Hence, eachcolour while passing through the prism bends at differentangles with respect to the incident beam. This gives rise to theformation of the colour spectrum.Can you say which colour undergoes maximum deviation?Violet light bends the most whereas red colour deviates least. However, Newton did not stop at this point. Hethought that if seven colours can be obtained from a white light beam, is it possible to obtain white lightback from the seven colours? For this, he placed an inverted prism in the path of a colour band. He wasamazed to see that only a beam of white light comes out from the second prism. It was at this point thatNewton concluded that white light comprises of seven component colours.Formation of a rainbowThe rainbow is a natural phenomenon in which white sunlight splits into beautiful colours by water droplets,which remain suspended in air after the rain.If we stand with our back towards the sun, thenwe can see the spectrum of these seven coloursSANDIP CHAKRABORTY. M: 9577556400,9435899571 Page 8
  9. 9. WISDOM ACADEMY 9 ARUNACHAL,KALI BARI BAZARDo you know why a rainbow is shaped similar to an arc?This is because the rainbow is formed by the dispersion of white light by spherical water droplets. It isthe shape of the water droplets that gives the rainbow an arc shape.A rainbow appears arc-shaped for an observer on ground. However,if he sees the rainbow from anairplane, then he will be able to see a complete circle. This is because he can observe the drops that areabove him as well as below him.A rainbow appears arc -shaped for an observer on ground.However, if he sees the rainbow from an airplane,then he will be able to see a complete circle. This is because hecan observe the drops that are above him as well as below him.Atmospheric RefractionFlickering of objectsObserve an object that is placed near a rising flame or fire. It will appear to be flickering. This isbecause the air above the fire is relatively hotter than the air further up in the atmosphere. Hence, hotair rises up and cold air moves in to fill the space. This process results in the variation of refractive indexof air, present in the vicinity of fire. The refractive index of hot air, which is rarer than cool air,is less than that of cool air. The physical condition of the atmosphere changes continuously, therebybringing a continuous change in the refracting index of air. Hence, the apparent position of the objectseems to fluctuate when seen through hot air.Q. Why do stars twinkle?Ans. Stars twinkle due to atmospheric refraction of starlight. As the stars are very far away,they behave as almost point sources of light. A son account of atmospheric refraction, the pathof rays of light coming from the star goes on varying slightly, the apparent position of the starfluctuates and the amount of starlight entering the eye flickers. So, sometimes, the starappears brighter and at some other time, fainter. Thus, the stars twinkle.SANDIP CHAKRABORTY. M: 9577556400,9435899571 Page 9
  10. 10. WISDOM ACADEMY 10 ARUNACHAL,KALI BARI BAZARQ. Explain why the planets do not twinkleAns. Planets are much closer to the earth and are seen as extended sources. So, a planet maybe considered as a collection of a large number of point-sized light sources. Although lightcoming from individual point-sized sources flickers but the total amount of light entering oureye from all the individual point-sized sources average out to be constant. Thereby, planetsappear equally bright and there is no twinkling of planets.Early sunrise and delayed sunsetAs viewed from the Earth, the sun rises 2 minutes before the actual sunrise and sets 2 minutes after theactual sunset. So, you see how we get to see sunrise 2 minutes before the actual sunrise. Similarly,after two minutes of sunset, we can still see the sun. Hence, atmospheric refraction lengthens a day by2 + 2 = 4 minutes every day.Scattering Of Light Why is the colour of the clear Sky Blue?The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength ofvisible light. These are more effective in scattering light of shorter wavelengths at the blue end than lightof longer wavelengths at the red end. The red light has a wavelength about 1.8 times greater than bluelight. Thus, when sunlight passes through the atmosphere, the fine particles in air scatter the blue colour(shorter wavelengths) more strongly than red. The scattered blue light enters our eyes. If the earth had noatmosphere, there would not have been any scattering. Then, the sky would have looked dark. The skyappears dark to passengers flying at very high altitudes, as scattering is not prominent at such heights.You might have observed that ‘danger’ signal lights are red in colour. The red is least scattered by fog orsmoke. Therefore, it can be seen in the same colour at a distance.Sunrise and sunsetAt sunrise or sunset, the sun is located near the horizon of the Earth. Hence, light has to travel a longdistance through the Earth’s atmosphere. At the time of sunrise or sunset, when white sunlight fallson suspended atmospheric particles, blue colour light scatters out in deep space, while red colour lightscatters less, and reaches the observer on the surface of the Earth. Hence, when this less scattered redlight reaches our eyes, the sun and its surroundings appear to be reddish.Q. W hy does the Sun appear reddish early in the morning?Ans. In the early morning, the sun is situated near the horizon. Light from the Sun passes throughthicker layers of air and larger distance in the earth’s atmosphere before reaching our eyes. Whilepassing through atmosphere blue light is most scattered away and the Sun appear reddish asshown in fig.SANDIP CHAKRABORTY. M: 9577556400,9435899571 Page 10
  11. 11. WISDOM ACADEMY 11 ARUNACHAL,KALI BARI BAZARTyndall effectSome natural phenomena are governed by the scattering of sunlight through suspended air particlespresent in it from random directions. Scattered sunlight may be white or of any component of the sevencolours, depending on the size of the particles that cause the scattering. This phenomenon is governedby the Tyndall effect.The Tyndall effect is caused by the scattering of light by very small air particles, which are suspendedin the Earth’s atmosphere. To observe the Tyndall effect, the particles diameter should be lessthan th of the wavelength of the light used. This effect can be seen when light enters through ahole in a dark room filled with dust particles. Have you looked at light rays coming through clouds,holes, or headlight beams during a foggy night? These are some well known examples of theTyndall effect.The colour of the scattered light depends on the particle size.(i) Fine particles mainly scatter blue light.(ii) Large particles scatter red light.It is observed that blue colour light scatters more easily than red colour light. This is because red colourlight is of a longer wave length.Do You Know:When there is no impurity present in air, the colour of the sun at sunrise and sunset appearsto be yellowish. Due to the presence of salt particles in air over seas and oceans, the colour of the sunat sunrise or sunset appears to be orange. Due to the presence of red iron-rich dust, the sky appearsred from the Martian surface. All these natural phenomena take place due to the scattering of sunlightThe least scattering red colour light finds its application in various fields. For example, in marking redlight, danger signals etc. red colour is preferred because it is scattered least by fog, smoke, and dustparticles present in air.If there was no atmosphere on the Earth, there would no scattering of light. Hence, in deepspace, the sky will appear to be dark.Q. Why does the sky appear dark instead of blue to an astronaut?Ans. Colour of the sky is on account of scattering of light of shorter wavelengths by particles inthe atmosphere of earth. If the earth had no atmosphere, there would not have been anyscattering and the sky would have looked dark. When an astronaut in his spacecraft goesabove the atmosphere of earth, sky appears dark to him because there is no scattering of light.When located overhead, why does not the sun appear reddish in colour?This is because light travels a relatively shorter distance when located overhead. Because of this reason,scattering of blue as well as red light is much less when the sun is near the horizon.Q 9 . A person needs a lens of power - 5.5 dioptres for correcting his distant vision. Forcorrecting his near vision he needs a lens of power + 1.5 dioptre. What is the focal l ength ofthe lens required for correcting (i) distant vision, and (ii) near vision?Ans. (i) Power of lens needed for correction distant vision of the person P 1 = -5.5 D Focallength of lens required for correcting distant visionf 1 = 1/p 1 = 1/-5.5 = -18cm(ii) For correcting near vision power required P 2 = + 1.5 DFocal length of lens required for correcting near vision f 2 = 1/p 2 = 1/-1.5 = 0.67m=66.7cSANDIP CHAKRABORTY. M: 9577556400,9435899571 Page 11