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LIGHT AND OPTICAL INSTRUMENTS, Light is a form of radiant energy which is, , Reflection, , transmitted through space without any, material medium. lt causes sensation of, other electromagnetic radiavision. Like all, transmitted in the form of, tions, light is, transverse waves. The speed of light in, vacuum, , is 3, , x, , made by, , man, e.g.,, kerosene lamp, etc., , ray, , ray, Normal, , Fig.1, , Sources of Light, many, There, source. There, natural, , Reflected, , Incident, , 10° m/s., , are, , by a Plane Mirror, , sources, are, , of light. Sun is a, several s o u r c e s, , Plane Mirror, , Image Formation by a, , candle, electric bulb,, uwiuwuwuy, , The bodies which give out light energy by, themselves a r e called luminous bodies., However; there a r e large number of substanthemsel, ces which do not give light energy by, , falling on, them. Such substances a r e furniture, house,, books, trees, etc. Even moon does not give, of, light of its own, but reflects light energy, ves, but reflect the, , light, , energy, , Sun., , The bodies which do not give, , of their own,, them, , are, , Mirror, , but reflect light, , light energy, , energy, , falling on, , called non-iuminous bodies., , Eye, , Fig.2, on a, Rays of light from a source O fall these, plane mirror and are reflected back;, , reflected rays enter the eye of the observer,, appearing to come from a point I behind the, mirror. The eye s e e s the, at this point I. However,, , of the s o u r c e, the light rays do, , image, as, , they, , not actually come from this point, the image, is called a virtual image. This kind of image, cannot be taken on a screen. When there is an, , medium in, turned back into the same, accordance with definite laws. This pheno, When a parallel, menon is called reflection., , formed. Areal image can be taken on a screen., Properties of Image formed by Plane Mir-, , Reflection of Light, When rays of light fall, , on a, , surface,, , are, , beam of light is incident on, , a, , polished surface,, , of light remains parallel,, reflection., Such a reflection is called regular, on a, incident, is, When a parallel beam of light, Lhe reflected beam, , beam, rOugh surface, the reflected, , is scattered, , And diffused, such a reflection is called ir, , regular reflection., Laws of Reflection, and, (i) The reflected ray, the incident ray, the normal at the point of incidence lie in the, Bame plane., (i) The angle of incidence (i) is equal to, , the angle of reflection (r)., , actual intersection of rays, a real image is, , ror, , (i) The image formed by a plane mirror is, virtual, erect and laterally inverted., ii) The image is of the same size as the, , object., (iii) Image and object are equidistant, from the mirror., , (iv) When the plane mirror is rotated, through a ceriain angle, the reflected ray, turns through double the angle., (v) The minimum height of the mirror to, , be required to see the' full image of an object, of height h is

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(vi) When an object moves with a velocity, image in the plane mirror moves with a, velocity 2., (vii) When two plane mirrors are inclined, U, its, , at an angle 0 and an object is placed between, them then the number of images formed is, , given by, 360, , 1 . In, , case, , 36, , metrically and, , the, , object lies unsym, , is odd, the number of, , of, , a, , two, , hollow, , types:, , glass, , on, , verging, , and convex. Silver, vering the, outside gives a concave or con, mirror and, the, , r)of the, , PHYSICS, , curvature, , of Curvature: It is the radius (Ror, , sphere of which, , the mirror is, , a, , part, , Principal Axis It is the line joining the, centre of curvature and the vertex of the mirror, Focus: If a beam of light is parallel to the, , Showing lateral inversion, , Fig.3, , plane, , a convex or, , of the sphere of which the spherical mirror is, , Radius, , When two, , silvering the, , glass from, diverging mirror,, Terms related to Spherical Mirrors, Pole or Vertex It is the centre (0), of a, spherical mirror., Centre of Curvature : It is the centre (C), inside gives, , part. A ray through the centre of, is reflected along the same path., , images formed is 360, 2012YH9, , sphere. Spherical mirrors are of, , concave, , principal axis and falls on a spherical mirror,, , mirrors, , are, , placed, , parallel to each other 0 becomes 0, thus, infinite number of images are formed., (viii) For a plane mirror, power is zero,, focal length is infinite and magnification is, , the rays after reflection either converge to a, point F (concave mirror) or appear to diverge, from the point F (convex mirror). The point P, is always situated on the principal axis and is, , called the focus., , one., , Example, An object is placed between two plane, mirrors inclined to each other at an angle of, 60. Find the number of images formed by, , these mirrors., , Solution, , Here,, , 60, n=, , 360, , ULLLLL, , 1, , 360- 1 =6-1-5, 60, , Fig.5. (a)Concave Mirror:Focusis Real, , Spherical Mirrors, Spherical mirrors are obtained by silver, ing a piece of glass which would form a part, Hollow glass sphere, , Convex, mirror, , Concave mirror, C, (Centre of, , curvature), , (Pole), , Principal axis, P, , (Pole), , Fig.5.(b)ConvexMirror:FocusisVirtual, (Radius of curvature), , Fig.4, , beam of light parallel to the principa, A, axis falling on the spherical mirrors:, OPole or Vertex

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F Focus, OF distance between, focal length, , pole and focus, , =X radius of curvature, , 2or, , radius, , r, , 2, , of curvature, , Sign Convention for Spherical Mirrors, Sign convention is used to assign either, a positive or a negative sign to all measurements of length., (i) All the distances are measured from, the pole of the mirror., (ii) Distances measured in the direction, , Mirror Formula, , of the incident light are taken as positive., , Mirror formula is=, , (ii) Distances measured in the direction, , Where,f= focal length of the mirror, distance of the image fromn the, , pole, , of the mirTor, u = distance of the object from the pole, of the mirror, , opposite to the direction of the incident light, are taken as negative., , (iv) The heights measured upwards and, perpendicular to the principal axis of the mirror are taken as positive while downward, distances are taken as negative., , Image Formation in Concave Mirror, , Fig. No, , Position of Object, , Position of Image, , At Focus (F), | Between F and C, , 6. (a), , At infinity, , 6. (b), 6. (c), , Beyond centre of curvature, , AtC, , 6. (d), , Between C and F, , AtC, Beyond CC, , 6. (e), , At F, , Atinfinity, , 6. (), , Between the pole P of the Behind the mirror, , Nature of lmage, Real and inverted, , Sizeof Image, , Highly diminished, , Real and inverted, , Diminished, , Real and inverted, , Same size, , Real and inverted, , Enlarged, Infinitely large, , or Real and inverted, , highly enlarged, , Virtual and erect, , Enlarged, virtual, , mirror and Focus F, Rays from the object, Rays from the, object at intinity, , C, |Image, , Objec, , (betweep, , Cand F), , Reflectedr a y s, , Reflected rays, , (Image iss, , formed at F), , (beyond C), , Fig.6.(d), , Fig.6.(a), Rays trom the object, , o, , o atF), , Object, Object, , (beyond C), , Image, , hected, rays (between C and F), , Reflected rays will, meet at infinity, , Fig.6.(e), , Fig.6.(b), , E, , Rays from the, , Object, , object, , Object, , ckat C), , Image, , F and P)S (appears, , Rays from the, , behind the, , Mirror, (at C), , EVirtual, , (between image, , mirror), , Fig.6. (), , Fig.6.(c), Images formed by a, , concave, , mirror

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Image, , Formation in Convex, , Pig. Position of PositionSize ofMirror, | Nature, No, of ImageImage, Behind Dimini-, , 7.a) Anywhere, , beyond the the, oe (P), , 7.(b, , shed, , mirror, Behind, , do-, , the, , mirror, , 7.(c-do, , mage|, , Rellecting, , and vir, , tual, , tual, , Behind, , Dimini- Erect, , the, , shed, , and vir, tual, , Refecte rays, , comingom the mimor, , and, , stores, , Erect, , Dimini- Erect, shed, and vir, , marror, , 3, , mszboptdheesiopes, Shaving and make | Vigilanee, mirrors, in big shops, up mirror, , 6., , tele, , scope, Radiation collector, in 8olar heating, , dovices, Refraetion of Light, , Light travels, , in, , straight line in, , a, , a, , homogeneous medium, but when light passes, from one medium to another, it is deviated, from its original pnth. This deviation of, , light, , is called refraction of light., , Refraction is essentially a surface, Object, , image, (appears to be formed behind, the mirror: erect, diminished,, , virtual), , phenomenon which occurs at the interface, between two media. When light travels from, a rarer medium (air) to an, optically denser, medium (glass, water), the ray of light bends, towerds the normal. Conversely, a ray passing, from a denser to a rarer medium bends away, , Fig.7.(a), , from the normal. The ray which is incident, , Refected rays, , normally does not suffer refraction and goes, , coming rom the mirror, , straight in the other medium., Image, , Object, , (appears to be formed behind, (the mirror: erect, diminished,, , virtual), , Incident ray, , Normal, N, , A, , Medium 1, , Fig.7.(b), Uses of Spherical Mirrors, , Refracted ray, N, , Rectangular, glass block, , Reflected rays, , N, , Medium 2, , Glass, , coming from the mirror, , Air, , N, , -, , Normal, , F, , Object, , Emergent ray, , Image (appears to be, , Fig.8.Bendingofraydue to refraction, , fomed behind the mirror, , erect, diminished, virtual), , Fig.7.(e), Concave Mirrors, 1., , Dentist's mirror, , side, , mirrors, , (driver's mirror), As, , reflectors, , in, , Staircase, , mirrors, , headlights of cars on double decker, , andsearchlights, , speed of light in vacuum, , to the speed of light in a, , Convex Mirrors, Rear-view mirrors, or, , Refractive Index, The ratio of the, , buses, , given medium 1s, , called the absolute refractive index of the, , medium., , Absolute Refractive Index of a medium, , Speed of light in vacuum, Speed of light in the medium, , Speed of light in vacuum is 3 x 10 m/s

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The absolute refractive indices of some, common substances are given in the following, , Hence,, , table, , Substance, , Absolute Refractive, , (Medium), , Index (n), , Air, Ice, Water, , 1.0003, 31, 1.33, 1.36, 1.43, 1.50, 1.52, , Alcohol, , Sulphuric Acid, , Benzene, , Crown glass, Diamond, 2.42, The Relative Refractive Index, When the light passes from medium 1 to, medium 2 then the refractive index ofmedium, 2 with respect to medium 1 is written as n12, and is defined as, , Speed of light in medium 1, n12Speed of light in medium 2, Laws of Refraction, , 1. The incident ray, the refracted ray and, the normal at the point of incidence all lie in, the same plane., 2. The ratio of sine of the angle of incidence to the sine of the angle of refraction is, this, a constant for a given pair of media and, constant is called the refractive index of the, This, second medium with respect to the first., constant is written as n12. Suppose the light, then the refractive, index can be written as naw which means, index of water with respect to air., , goes, , from air to, , glass,, , refractive, , i = 90, r =c, , sini, , sin 90, , sin r, , Sin c, =, , or, , =, , n12, , n12, , Sin c, , c critical angle, Totally reflecting prisms are used in peri-, , scopes and in binoculars. The principle of total, internal reflection is also used in optical, , fibres. It also explains the natural phenomena, of mirage and looming., This phenomena is observed in, a desert during hot summer days. The mirage, , Mirage, , is the deceptive appearance of an object, caused by the bending of light rays (refraction) in layers of air of varying density. In, deserts air near the ground gets heated intensely by sunshine but it is cooler as we go up, above the ground. This results in decreased, density near the ground and increase in den-, , sity at higher positions. When a beam of light, travelling form the top of a tree downward, (where air is denser) enters a rarer layer, it, , is, , refracted away from the normal. With each, Successive layer of air, the angle of refraction, , increases and ultimately a state is reached, when the angle of incidence becomes greater, than the critical angle between the two layers., , Then the incident ray is directed upwards and, , suffers total internal reflection. This ray, on, reaching the observer's eye, creates an inverted image of the tree and the sand apPpears, to be a pool of water in which an inverted, , image of a, , tree at its bank, seems to have been, , sini = a constant = n12, , formed., , sin r, , In the very cold polar regions of theearth, Looming occurs when air closer to theground, is much colder than the air above. The rays, , Total Internal Reflection, When, , angles of indenser medium (such as glass), , light passes at, , small, , Cidence from a, as air), the, o, less dense medium (such, a, from the normal, ray deviates away, , refracted, , in the less., in the, If the angle of incidence of light, a particular, denser medium is greater than, eritical angle for that, angle (namely, theis not refracted into, the, medium), the light, back into, reflected, is, totally, but, rarer medium, This is called the total, the denser medium.The, critical angle o c c u r s, internal reflection., of refraction is 90., when, , angle, , an object Gust as a ship) on earth keep, from, on bending in such a way that they enter the, , observer's eyes above the line of sight. The, objects on the ground seem to be floating, upside down in the sky., Optical Fibres Optical fibres are long, fibres of high-index glass cladded with a thin, layer of lower index glass, which are arranged, side by side in a definite order. The light, entering from one end of the fibre is transmitted along its length without any loss, with, thousands of successive internal reflections.

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Hence, an image projected upon one end ofthe, , fibre is transmitted to the other end, where, the image can be viewed or photographed., A very recent use for the optical fibres is, in telecommunications, for carrying pulses of, , light from a laser which represent informa, tion such as telephone conversations, television pictures and computer data. An optical, fibre has a much greater information-carrying capacity than a copper cable of the same, an electrie current. Hence, the optical fibre has replaced the metal cables, , thickness carrying, , 0, , Plano, , Double, , Plano, , Double, , convex, , convex, , concave, , lens, , lens, , lens, , Concave, lens, , Fig.9.(b)Types ofLenses, Convex lens, or, , converging lens, , Parallel rays, of light, , in the telecommunications equipments., , Optical Lenses, A lens is defined as a "portion of a, transparent refracting medium bounded by, wo surfaces which are generally spherical"., There are two types of lenses convex and, concave. A convex lens is thickest in the middle and thinnest at its periphery while a con-, , Principal, , axis, , (Focus), , (Focus)-, , Focal length), , Fig.10.(a), Convex lens, , cave lens is thickest at the periphery and, thinnest at the middle., , Important Terms regarding lenses, Aperture: The diameter of a lens is called, aperture of the lens., , Optical Centre (O): This is the point, within the lens through which a ray of light, , diverging lens, , Parallel rays, of light, , Principal, axis, , F, , F, , ****, , (Focus), , - (Focus)-, , passes undeviated., , Principal Focus: Light falls on a lens, after, , undergoing refraction, it either converges toa, point (in convex lens) or appears to diverge from, , (Focal length), , Fig.10.(b)Aconcavelens divergesaparalle!, beamoflightrays, , a point (in concave lens). This point is called, , principal focus, fig. 9 and 10., The distance between the, Focal, Length:, optical centre and the principal focus is known, as focal length., , Power ofa Lens I t is the reciprocal of, focal length. If the focal length is measured in, metre, the power of the lens is in dioptre., Sign Convention for Lenses, 1. All measurements are taken from the, , Surface 2, , Surface 1, , C2 Principal, , Axis, , Fig.9.(a), , optical centre of the lens., 2. The distances in, , direction perpen, dicular to the principal axis are measured, from the principal axis., 3. The distances measured in the direc, a, , tion of incident rays are positive., 4. The distances measured in the direction opposite to that of the incident rays are, , negative.

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mage Formed by Concave Lens, , Fig. No, , Position of Object, , Position of Image, , 12. (a), 12. (b), , At infinity, , 12. (c), , Beyond 2F2, , Same side, Same side, , At 2F2, , Same side, , 12. (d), , Between F2 and 2FF2, , 12. (e), , At F2, , 12. (), , as, as, as, , | Same side as, , Size, , the, the, , object, , the, , object, , Same side, , as, , Nature of Image, , Very snall, Smail, Diminished, , Virtual and erect, Virtual and erect, Virtual and erect, , Diminished, Diminished, , Virtual and erect, , theobject Diminished, , Virtual and erect, , object, the object, , Same side as the object, , Between F2 and O, , of Image, , Virtual and erect, , focus)., **********, Rays (image appears at, coming, , from, , infinity 2F2, , F2, , *****wewuu*w*****., , Image, , 2F, (Object, between, , 2F2 and Fa, , Fig.12.(a), , 2F2, (Object behind 2F2), , Fig.12.(d), , F 2F1, , F2 (Image, between, , 2F2, , F2, , (Object, , F2 and O), , at F2), , Fig.12.(e), , Fig.12.(b), , 2F2, , F2 Image, , 2F2, (Object, , Image, , Image, F2, (Object, , o, , between, , at 2F2), , F2 and O), , Fig.12.(0D, , Fig.12.(c), , Magnification, , Produced, , by Mirrors and, , Lenses, , Magnification (M), , Height of theimage (h), Height of the object (h), , M=h, , The formula for magnification in mirrors, carries a negative sign whereas in lens it, carries a positive sign., Power of a Lens, Power ofa lens, , h, , -D, , Focal length in metre, , Distance of the image mirror/lens, , where D is dioptre, which is a unit ot, , Distance of the object mirror/lens, M=, , power of lens,, , 1 D= 1, , m"

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Uses of Lenses, , Dispersion and Colour, , Convex Lens, 1., , In the eye (variable, focal length), In the projector, , 2, , In the camera, , 3., , Concave Lens, As, , a, , wide, , angle, , Dispersion The splitting of a beam of, white light into different components when it, , spyhole in doors, , passes through a transparent medium, such, , As glasses to cor, rect short sight, As a wide angle lens, in coach rear win, , as a glas8 prism is called dispersion of light., , Dispersion occurs because light waves of dif, , ferent wavelengths are refracted at different, angles. This occurs because the refractive, , dows, , index of glass is different for different colours,, , In the telescope ob-, , As an eye lens in, , jective, , Galilean telescope, , the refractive index is greatest for violet light, and least for red light. The rainbow is the most, commonly observed example of dispersion., , In the microscope, As glasses to cor-, , The light splits into seven colours, as, , rectlong sight_, , follows, , Microseope, , V, , Microscope is an instrument for producing enlarged images of small objects. The, simple microscope or Magnifying glass has, , a single convex (coverging) lens. The compound microscope has essentially two lens, , systems and gives a much greater magnifica-, , tion. The lens (objective lens) facing the object, has a very short focal length and forms a, magnified real image ofthe object. The second, lens held close to the eye is called the eye, , piece. The eye piece which acts like a simple, the, magnifying lens then further magnifies, This, , lens., image formed by the objective, the total magimage is a virtual o n e . Thus,, and, nification is the product of the objective, a, magnification. There is usually, piece, eye, , Violet, , Y, G, B, Blue Green Yellow, , I, Indigo, R, , Orange, , Red, , This coloured band is called spectrum. It, is observed that red light is deviated the least,, the violet is deviated the most., , Colours of Objects: An opaque object, reflects certain colours and absorbs the rest., , The colours that get reflected give, , the colour, , of the object. The green leaves appear green, absorb, as they reflect only green colour and, all the other colours of the spectrum., Primary colours are those colours which, cannot be obtained by mixing any other, , obtained by mixing, n, o De, colours., Red, Blue and Green are primary, , choice of objective on a compound microscope, , colours. When mixed in specific proportion, , magnification., giving low, medium, and high, , they produce white light., , Telescope, Telescope is, , an, , optical, , instrument used, , It condetect or' examine distant objects., mirrors capable of producSIsts of lenses and, more, image and of collecting, a, , to, , ng magnified unaided, , ght, , than the, , eye. The, , of a, , refracting, , tube with, , a, , telescope essentially consists from a distant, at each end. Light, which, cas system strikes, the objective lens, , Secondary, , re, , of a telescope, objective. Distant objects, its, diameter, the, of, In a, of, of parallel rays., stars emit a bunch, e, has a longer focal, the objective lens, , The size, , elescope, ength than, , n, , lens in microscope., the objective, lens forms a real, objective, the, telescope, The eye-piece, of the distant object., , mage, , 1orms a virtual image., , are, , those colour that, , produced by mixing two primary colours., , Yellow,, , Magenta and Cyan (peacock blue) are, , secondary colours., , Complementary Colours : Complemen, obtained when a primary, to, colour is mixed with a secondary colour, , tary colours, , are, , form white., primary, magenta, , red, blue, green, , object first, , at its focal point., terms, 9roduces an inverted image, is measured in, , colours, , red +blue, blue + green, , = eyan, , red +green, red, , colours, , secondary, colours, , =yellow, blue, , +green +, , red +cyan, , -, , =, , =, , white, , white, , green+ magenta =white, blue + yellow =white, , complemen-, , tary colours

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Rainbow, , Rainbowis observed when we stand with, , back towards the sun. It occurs due to, refraction and internal reflection of sun's rays, by the rain droplets in the sky. A rainbow is a, coloured arc, comprising of seven, violet, indigo, blue, green, yellow, coloursand, orange, , our, , red. The coloured are is the result of refraction, and internal reflection, of the light rays that, enter the raindrop, each colour, being deviated, at a slightly different, Due, to this each, angle., of, incident light will be separated upon, ray, , the drop. Light, emerging, trom, emerging from, the, , raindrop has minimum deviation of about, , 138 forming a cone with angular radius of, bout 42, with arcs from violet to red, as we, from inside to outside of rainbow., cattering of Light, When light rays strike atoms, molecules,, ove, , any other, , or, , tiny particles, the light rays, , are, , deviated in a new direction. This is known as, scattering of light. The most common example, of scattering of light is the blue sky. The air, molecules scatter more blue rays than any, , other rays.. Violet and blue have shorter, , wavelengths, than the othercolours and are, scattered most. It is, called Rayleigh scatter, , ing. The amount of scattering is proportional, , to, , where, , is the wavelengthof the light., , applied to a camera objective, it improves the, quality and brightness of the image by cutting, out the, , reflections from the lens, surfaces., , The Human Eye, cm, , The human eye is, diameter and its, , in, , spheroidal about, , 2.5, , spherical shape, , near, , maintained by the pressure of the fluid, inside its cavity. A, watery fluid (aqueous, humour) in front of the lens and a transparent, jelly (vitreous humour) behind it contribute to, the refraction producing an image on the, retina which is an optically sensitive rear, is, , surface., , Although, , the, , image on the retina is, inverted, our brain has learned to interpret it, right side up. The retina contains millions of, delicate nerve endings whose electrical pulses, are carried to the brain. The iris is a, variable, sized, , diaphragm controlling, , the, , amount of, , light entering the eye., , Main Parts of the Eye, , Cornea-curved transparent membrane, at the front of the eye;, , light, , enters the eye, , through the cornea; most of the refraction, takes place inside cornea., , Iris-coloured part of the eye; controls ad, justs the proper amount oflight entering the eye, , Pupil-an aperture in the iris; its diameter, , changes from about 3 mm to about 8 mm, , depending on the intensity ofthe incident light., In bright light for example, the iris makes the, , It holds only for scattering very fine particles., , pupil contract, thus decreasing the amount of, , Interference of Light Waves, Light waves interfere with each other, , light entering the eye, whereas in darkness it, dilates to allow more light, enter the eye., , when they cross through the same spot. A, wave of light is a transverse wave and hence, it has crests and troughs. The crest of one, wave adds to the crest of another wave, when, , Lens-focuses the image on the, retina., Retina-light sensitive surface on which, image of the object is formed. It also has rods, , they superimpose each other and this is called, , constructive interference. This gives brighter, light than either wave would have given, separately. In the case of the crest of one wave, , choroid, , ing each other, the trough reduces the height, of the crest, leaving a dim or dark spot. This, called destructive interference., , fovea-, , is covered with a chemical film ofjust the right, thickness to stop most of the light that would, ordinarily be reflected and cause glare. When, , cornea, , Vitréous, Humour, , retina-, , non-reflecting coatings for glass. The surface, , eye lash, conjunctiva, , and the trough of another wave superimpos, , A useful application of interference is in, , eye lid, , sclerotic, , pup, , -lens, , aqueous humour, , blind spot, , suspensory ligaments, ciliary muscle, , optic nerve, , aye muscle, , yellow spot, , Fig.13

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(sensitive to finer details of the objects) and, cones (tor detectio, , of colour of object), , The property of eye lens to change and, adjust its focal length for seeing objects clear, , ly at difterent distances is called its power of, , accommodation. Due to the accommodation, property ot the eye, it can change its lens, , thickness with the help of ciliary muscles,, , Fig.15, , from 3.6 mm to 4.0 mm., , Yellow spot : It is the spot on the retina, that is the most sensitive to light., , Blind spot: It is the spot where the optie, nerve enters the eye. It is insensitive to light., , Least distance of distinct vision A nor, mal eye should be able to see the objects, (the fur point) down to, clearly, from, infinity, about 25 cm (the near point). The minimum, distance of an object from the eye at which it, , Hypermetropia (Longsightedness): In, , this defect, distant objects are seen clearly, whereas nearby objects appear to be blurred., This is caused by the condition in which the, , eyelens becomes small in size. This generally, , happens in the old, , age because the muscles, , that changethe shape of the eyelens get weak, and do not function properly. In this case the, image is formed behind the retina (Fig. 16),, , can be seen clearly by the naked eye is 25 cm., The size of an object appears to be maximum, when seen at the least distance of distinct, , vision. The lens is thin when seeing distant, objects and thick when seeing near objects., The maximum thickness is. 4 mm, when, seeing objects at a distance of 15 em. Objects, nearer than 15 cm cannot be seen clearly, as, their image cannot be focussed on the retina., The image of the object seen is most, when it is at a distance of 25 cm. Hence, the, , distinet, , Fig.16, Hypermetropia can be corrected by using, a convex (or convergent) lens (Fig. 17)., , least distance of distinct vision is 25 cm., , Defects of Vision, , Myopia (Shortsightedness): Aperson suf, fering from myopia cannot see distant objects, this, , clearly with the naked eye. The cause of, , defect is that the lens of the eyes becomes, elongated. The image, instead of being tormed, , at the retina,, (Fig. 14)., , is formed in front of the retina, , Fig.17, Astigmatism : In an eye suffering from, this defect, the front surface of the eye ball is, , Retina, , Fig.14, , Myopia can be.corrected by using a, , Cave (or divergent) lens (Fig. l5)., , con-, , not curved equally in all directions like a, , sphere and this produces indistinct images., The images are formed at varying distances, from the retina. This defect is corrected by, wearing spectacles with eylindrical lenses., Persistence of Vision An image lasts on, the retina for about one-tenth ofa second after, the object has disappeared. This effect makes, possible the production of motion pictures., wenty-four separate pictures, each slightly

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different, on, , to the, , from the, screen, , previous one,, , per, , projected, second and give the imare, , pression ofcontinuity. In a television receiver, , twenty-five complete pictures are produced, every second. This persistence of vision is, basically due to the inertia of the brain.