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LIGHT AND OPTICAL INSTRUMENTS, , —, Light is a form of radiant energy which is, transmitted through space without any, material medium. It causes sensation of, yision. Like all other electromagnetic radiations, light is transmitted in the form of, transverse waves. The speed of light in, vacuum is 3x 10° m/s., Sources of Light, , There are many sources of light. Sunis a, natural source. There are several sources, made by man, e.g., candle, electric bulb,, kerosene lamp, etc., , The bodies which give out light energy by, themselves are called luminous bodies., However; there are large number of substances which do not give light energy by themselves, but reflect the light energy falling on, them. Such substances are furniture, house,, books, trees, etc. Even moon does not give, light of its own, but reflects light energy of, sun., , The bodies which do not give light energy, of their own, but reflect light energy falling on, them are called non-iuminous bodies., Reflection of Light, , When rays of light fall on a surface, they, are turned back into the same medium in, accordance with definite laws. This phenomenon is called reflection. When a parallel, beam of light is incident on a polished surface,, the reflected beam of light remains parallel,, such a reflection is called regular reflection., When a parallel beam of light is incident on a, rough surface, the reflected beam is scattered, and diffused, such a reflection is called irregular reflection., , Laws of Reflection, , (i) The reflected ray, the incident ray and, the normal at the point of incidence lie in the, ‘ame plane., , (ii) The angle of incidence (i) is equal to, the angle of reflection (7)., , Reflection by a Plane Mirror, , Reflected, ray, , Normal, Fig.1, , Image Formation by a Plane Mirror, t, , , , Eye, . Fig.2 ., , Rays of light from a source O fall on a, plane mirror and are reflected back; these, reflected rays enter the eye of the observer,, appearing to come from a point I behind the, mirror. The eye sees the image of the source, at this point I. However, as the light rays do, 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, actual intersection of rays, a real image is, formed. A real image can be taken on a screen., Properties of Image formed by Plane Mir, 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 cercain 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 a

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. (vi) When an object moves with a velocity of a hollow sphere. Spherical mirrors ate», v, nig in the plane mirror moves witha two types : concave and convex. Silvering @, veloci i, , glass on the outside gives a concave or, (vii) When two plane mirrors are inclined verging mirror and silvering the glass, at an angle @ and an object is placed between _inside gives a convex or diverging mirr, , them -oieg the number of images formed is Terms related to Spherical Mirrors, Riven by :, , 360 Pole or Vertex : It is the centre (0) ts, aiid 1. In case the object lies unsym- spherical mirror., , fro, , . Centre of Curvature : It is the centre Cu, metrically and 360° is odd, the number of of the sphere of which the spherical mirror,, , 6 a part. A ray through the centre of curvatury, images formed is ee is reflected along the same path., , Radius of Curvature : It is the radius (Ror, r) of the 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, Showing lateral nversion Focus : If a beam of light is parallel to the, . Fig.3 principal axis and falls on a spherical mirror,, When two plane mirrors are placed the rays after reflection either converge to a, parallel to each other @ becomes 0, thus, in- _ point F (concave mirror) or appear to diverge, finite number of images are formed. from the point F (convex mirror). The point P, (viii) For a plane mirror, power is zero, __ is always situated on the principal axis andis, focal length is infinite and magnification 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, , 2012YH9 PHYSICS, , these mirrors., Solution, Here, 8 = 60°, n= ae z, 8, = a -1=6-1=5 Fig.5.(a) Concave Mirror:FocusisReal, Spherical Mirrors, , Spherical mirrors are obtained by silvering a piece of glass which would form a part, , , , Fig.5.(b) ConvexMirror:Focusis Virtual, , A beam of light parallel to the princip*!, axis falling on the spherical mirrors :, O — Pole or Vertex

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F — Focus, OF = distance between pole and focus, = focal length, , = : x radius of curvature, . f= g or t, Mirror Formula, Mirror formula is A 4 + 1.2, fovur, , Where, f = focal length of the mirror, , v= distance of the image from the pole, of the mirror, , u = distance of the object from the pole, of the mirror, , re radius 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, of the incident light are taken as positive., , (iii) Distances measured in the direction, 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 | Size of Image _ Nature of Image |, , | 6.(a) | Atinfinity | At Focus (F) Highly diminished Real and inverted |, , | 6.(b) | Beyond centre of curvature | Benee FandC | Diminished Real and inverted, , | 6c) ALC ALC Same size Real and inverted |, 6.(d) | Between C and F nd C Enlarged | Real and inverted, , | 6.(e) |AtF At infinity Infinitely large or Real and inverted, , highly enlarged, | Enlarged, virtual Virtual and erect |, , | mirror and Focus F, , , , Fig.6.(c), , 6.0 | Between the pole P of the Behind the mirror, , , , Fig.6.(d), ° car F), , F, , Reflected rays will, meet at infinity, Fig.6.(e), , , , Images formed by a concave mirror

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__ Image Formation in Convex Mirror, , , , Pe] Position of | Size of | Nature, No. | Object of Im | Image [of Image, 74a) | Anywhere I Rehind | Dita: t oe, bevond the | the | shed and vir, » pale (P) mirror | tut, 7. 1b) —do— Behind | Dimini- Krect, the shed and virmirror | | tual, 7. (ce) | —do— Behind | Dimini- | Breet, the shed and vir_mirror | tual, , , , , , the mirror : erect, diminished,, virtually, , Fig.7.(a), , ], , , , Fig.7.(b), , Uses of Spherical Mirrors, Reflected rays, , , , , , , , Fig.7.(c), { Concave Mirrors | Convex Mirrors ], 1 Dentist’s mirror | Rear-view mirrors, or side mirrors, , (driver's mirror), reflectors in | Staircase mirrors, headlights of cars on double decker |, and searchlights buses J, , a Shaving and make. | Vigilance mi, , up mirror in big shopr, atoren aa, , 4. | Reflecting tole, scope, 5 Radiation collector, , in solar heating, , devices, Refraction of Light, , Light travels in a straight line in a, homogeneous medium, but when light passes, from one medium to another, it is deviated, from its original path, This deviation of light, is called refraction of light., , Refraction is essentially a surface, 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, from the normal, The ray which is incident, normally does not suffer refraction and goes, straight in the other medium., , , , , , , , , , , , N,' ~ c, Normai, Fig.8.Bendingofrayduetorefraction, , Refractive Index, , The ratio of the speed of light in vacuu™, to the speed of light in a given medium ', 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, , table: _, , Substance Absolute Refractive, | ___ Medium) | Index (n), Air | 1.0003, | Ice 1.31, Water j 1.33, | Alcohol 1.36 |, | Sulphuric Acid 1.43 |, Benzene 1.50 |, | Crown glass 1.52, | Diamond See- ut], , The Relative Refractive Index, , When the light passes from medium 1 to, medium 2 then the refractive index of medium, 2 with respect to medium 1 is written as ni2, and is defined as, , Speed of light in medium 1, , = Speed 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, a constant for a given pair of media and this, constant is called the refractive index of the, second medium with respect to the first. This, constant is written as n12. Suppose the light, goes from air to glass, then the refractive, index can be written as maw Which means, refractive index of water with respect to air,, , nt =a constant = 712, J, , Total Internal Reflection, , When light passes at small angles of incidence from a denser medium (such as glass), to a less dense medium (such as air), the, refracted ray deviates away from the normal, in the less., , If the angle of incidence of light in the, denser medium is greater than a particular, angle (namely, the critical angle for that, medium), the light is not refracted into the, rarer medium but is totally reflected back into, the denser medium. This is called the ‘otal, internal reflection. The critical angle occurs, when angle of refraction is 90°., , , , , , , , , , Hence, i=90',r=c, sini _ sin 90°, sinr sine, , , , 1, or ———-=n12, sinc, c = critical angle, , Totally reflecting prisms are used in periscopes 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., , Mirage : This phenomena is observed in, a desert during hot summer days. The 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 density 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 appears, to be a pool of water in which an inverted, image of a tree at its bank, seems to have been, formed., , In the very cold polar regions of the earth,, Looming occurs when air closer to the ground, is much colder than the air above. The rays, from an object (just as a ship) on earth keep, 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.