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CHAPTER 4, , Reflection of Light, , Learning Objectives, , ¢ Reflection of light, laws of reflection, , Spherical mirrors, terms related to spherical mirrors, Rules for obtaining images formed by spherical mirrors,, , ray diagrams for the formation of different type of images, , by a concave mirror, formation of images by a convex, mirror, , *, , °, , New Cartesian Sign Convention, mirror formula,, magnification, , en we enter a dark room, we cannot see, , anything even though we keep our eyes open., As soon as we switch on a bulb ora tube light, most of, the objects in the room become visible to us. Why? ‘The, bulb or the tube light when switched on gives out an, energy called light. When this light falls on the objects, in the room, it is reflected back from their surfaces., When this reflected light enters our eyes, the eyes sense, it and send a message to the brain. The brain converts, this message into images of the objects. Thus, we can, see things around us only in the presence of light (Light, is a form of energy that produces the sensation of, , sight., , Luminous and non-luminous objects, Basically, we can see an object if it emits or reflects light., Certain objects such as the sun, a burning candle and, the hot filament of an electric bulb give out light and, cause the sensation of sight. Objects which emit light, - of their own are called luminous objects (Fig, 4.1)., Objects such as book, table, chair, plants and many, other things do not emit light. Objects which do not, , , , SE pight behaves like waves on Mondays,, Wednesdays and fridays, like particles, on Tuesdays, Thursdays and Saturdays,, , and like nothing on Sundays. 99, — WILLIAM BRAGG, , emit light of their own are called non-luminous, objects (Fig. 4.2)., , ‘ ee, +, Ws, aa, , sun burning candle _ lighted electric bulb, Fig. 4.1 Luminous objects, , , , , , Fig. 4.2 Non-luminous object, , Ray and beam of light, , An object which emits light is called a source of light., All luminous bodies are sources of light., , 1, Ray of light: A ray of light is the direction of, path followed by light emitted by a source. It is, represented by a straight line with an arrowhead, showing the direction in which ‘it is travelling, (Fig. 4.3a).
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w, a, , REFLECTION OF LIGHT, , , , , , , , , , Nature of light, , The controversy over the nature of light is one of, , the most interesting controversies in the history of, science. Many theories about the nature of light were, developed through experiments and by observation of, its behaviour., , Sir Issac Newton put forward his theory of light in, 1678 to explain reflection, refraction and rectilinear :, propagation of light. According to Newton's Corpuscular, Theory, light consists of tiny particles called corpuscles :, which are shot out by a luminous object., , Acconflicting theory of light was proposed by the, Dutch scientist, Huygens. According to Huygens, light, was a longitudinal wave. In 1801, Thomas Young, experimentally proved the wave-like character of light., This was a very strong point in favour of the wave, theory., , In 1860, Maxwell proposed that light was an, electromagnetic wave. Maxwell's theory was confirmed :, experimentally by Hertz in 1886. i, In 1905, Albert Einstein proposed a new theory, , of light called Particle Theory in order to explain, phenomena like photoelectric effect and atomic, excitation. According to Quantum Theory, light is, transmitted as tiny bundles of energy called photons., Some experiments show that light behaves like an, electromagnetic wave and others indicate that it has i, particle-like nature. Physicists have finally come to the i, conclusion that light has a dual nature, the particle, nature and the wave nature., , , , b. Parallel beam of light, , a. Ray, Fig. 4.3 Ray and beam of light, , 2. Beam oflight: A bundle (ora group) of light rays, emitted by a source of light and moving in the same, direction, is called a beam of light (Fig. 4.3b)., , REFLECTION OF LIGHT, , If we throw a tennis ball on a wall, the ball bounces, back. This means that when the tennis ball strikes the, wall, the wall sends it back. Similarly, when light falls on, a smooth and highly polished surface such as a mirror, or ashiny stainless steel surface, then it sends the light, , back., , If we hold a plane mirror in front of us,, , , , , , , , Reflection is the phenomenon in whi, rays on striking a polished smooth Surface such, a mirror are sent back into the same medium, *, , The amount of light that is reflected depends o, material and nature of the surface on which em, falls. An opaque body absorbs a portion of the li a, it allows some of the light that falls on it to Pass he ., and reflects the remaining light. A smooth and hi =, polished surface such as a mirror or a shiny aie., , steel surface reflects almost all the light that falls on it,, , ch light, , Regular and irregular reflections, , % . We see, the image of our face in it. This is because the, , entire light that falls on its surface is reflected in g, definite direction. Such a reflection is called a Tegular, reflection. The phenomenon due to which a paralle], beam of light travelling through a certain medium on, striking a smooth, highly polished surface (such as a, mirror) bounces back from it as a parallel beam of light, in some other direction is called regular reflection of, light. Regular reflection is useful in the formation of, images (Fig. 4.4a)., , If we hold a cardboard in front of us, we will notbe, able to see our face in it. This is because the light that falls, on its surface is reflected in all directions irregularly, in, a scattered manner. Such a reflection is called irregular, reflection. The phenomenon due to which a parallel, beam of light travelling through a certain medium, on striking a rough surface (such as cardboard), gets, reflected in various directions in the same mediumis, called irregular reflection of light (Fig. 4.4b)., , To study the reflec. 3n of light from, , a plane mirror, , A plane mirror is generally made from a thin flat glass, plate. One surface of the glass is polished to a high, degree of smoothness, forming the front surface ofthe, mirror, The back surface is silvered, i.e. painted witha, , , , b. Irregular reflection «, , a. Regular reflection, , Fig. 4.4 Types of reflection
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silver compound. The silver Coatin,, layer of some dark paint., , using a light box (also known as ra, When light falls on the smooth fr, it enters the glass plate and falls o:, , : Red eyes in flash Photographs, , ! You may have noticed Photo;, with their eyes glowing red,, , : photographic flash device is Used and it j, , : lens of the camera, Light fro is close to the, , pel, , , , , , , , , raphs of individuals, This is the Case when, , M the flash enters the, , ig is protected bya, , A beam of light is produced in the laboratory by, , Y streak apparatus),, ‘ont surface, most of, n the silvered surface,, , The light gets reflected from there and comes out of the, , front surface (Fig, 4.5). In diagram, hatching below the, smooth surface represents silvering., , Now let us understand some important terms, , related to the reflection of light., , 1., , Incident ray: The ray of light which falls on the, mirror’s reflecting surface is called the incident, ray. In Figure 4.5, the ray oflight AO coming from, the light box falls on the mirror surface, therefore,, AO is the incident ray. The incident ray always goes, towards the mirror., , Point of incidence: The point at which the, incident ray strikes the reflecting surface of, the mirror is called the point of incidence. In, Figuré'4.5, the incident ray AO strikes the reflecting, surface of the mirror at point O, therefore, O is the, , , , , light box, (source of, light), / °, os point of, incidence, , Fig. 4.5 Reflection of light from a plane mirror, , point of incidence,, , 3. Reflected ray: The ray of light which is sent, back by the mirror is called the reflected ray., ‘The reflected ray travels in the same medium in, which the incident ray is travelling. The reflected, “Tay always goes away from the mirror. In this case,, OB is the reflected ray., , Normal: The ‘normal’ is a line drawn at right, angle (perpendicular) to the mirror surface at, the point of incidence. In Figure 4.5, the dotted, line ON is the normal to the mirror surface MM’, at the point of incidence O., , 5. Angle of incidence: The angle which the, incident ray makes with the normal at the point, of incidence is called the angle of incidence. In, Figure 4.5, the angle AON is the angle of incidence., It is represented by Zi., , Angle of reflection: ‘The angle which the, reflected ray makes with the normal at the point, of incidefce is called the angle of reflection. IA, , Figure 4.5, the angle NOB is the angle of reflection., It is represented by Zr., , 4., , 6., , Laws of reflection, , When light falls on a smooth reflecting surface, eg., , mirrors, it obeys the following two laws of reflection:, , 1, The angle of incidence (Zi) is equal to the angle, of reflection (Zr), ie. Zi = Zr., , 2. The incident ray, the normal to the reflecting, surface at the point of incidence and the, reflected ray, all lic in the same plane. s, These laws of reflection are applicable to all types, , of reflecting surfaces (plane mirror, spherical mirror or, , diffused reflector such as a piece of Paper)., , When a ray of light is incident normally (ie. at right, angle) on a mirror, the incident ray coincides with the, normal, ., , So, angle of incidence, Zi = 0., , According to the first law of reflection, Zi= Zr., , So, angle of reflection, Zr = 0,, , , , ; Evenin diffuse reflections the laws of reflection are, : obeyed. The angle of incidence and so the angle of, , reflection varies from point to point as the surface is, not perfectly smooth,, , , , 4H9N1 40 NOILD3 1498
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Therefore, the reflected ray will also travel along the, normal. Thus, when a ray of light is incident normally, on a mirror, it retraces its path on reflection., , Lateral inversion, , If we look at our own image in a mirror, it looks, just like us and is of the same size. Have you ever, noticed that the image in a plane mirror is different, from you? If you move your right hand, you will, find in the image as if you have moved your left, hand (Fig, 4.6a). In other words, the left of the object, becomes the right of the image and vice versa. This, phenomenon is called lateral inversion. , It is due to lateral inversion of the image formed in, a plane mirror that it becomes difficult to read a text, page which is reflected by a plane mirror. The word, AMBULANCE on the hospital vans is written in the, form of its mirror image as AOVMAIUEMA. Do you, know why?, , Characteristics of an image formed, , by a plane mirror, , 1. The image is formed behind the mirror and has the, same size as the object (Fig. 4.6b)., , 2. The image is laterally inverted., , , , object, , Fig. 4.6 Characteristics of an image formed by a plane mirror, , 3. The image is as far behind the‘mirror as the object, is in front of it., , 4. The image is virtual. It cannot be received’on a, screen., , 5. The image is erect.
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SPHERICAL MIRRORS, , We have learnt about reflection of light from plane, surfaces, viz, a plane mirror, in the previous section, Not all mirrors that we use are plane. Even polished, and smooth curved surfaces can act as mirrors. For, example, the rear-view mirrors in cars and scooters, have curved surfaces,, , A spherical mirror is obtained by taking a part, (thin) of a hollow sphere of glass or any polished, metal. The reflecting surface of a spherical mirror may, be curved inwards or outwards. ., , 1, Concave mirror: A spherical mirror whose, reflecting surface is curved inwards, ie. faces towards, the centre of the sphere is called a concave mirror., , In Figure 4.7, side A acts as a reflecting surface and, , the other side B is made opaque (shown shaded)., , reflecting surface, , (reflection of light, takes place at, , concave surface), , hollow glass sphere +, , , outer surface made, , opaque by silver, , coating protected, by lead paint, , Fig. 4.7 Concave mirror, , TABLE 4.1 Differences between concave mirror and c, , , , , reflecting surface inner surface made, , (reflection of light ™. ~ opaque by silver, takes place at Coating protected, convex surface) by lead paint, , {hollow glass sphere, , Fig. 4.8 Convex mirror, , 2. Convex mirror: A spherical mirror whose, reflecting surface is, curved outwards ‘is called, a convex mirror. In Figure 4.8, side B acts as a, reflecting surface and the other side A is made, opaque (shown shaded)., ‘The differences between concave mirror and convex, mirror are given in Table 4.1., , Terms related to spherical mirrors, , Let us recognize and understand the meanings of the, following terms commonly used in discussions about, spherical mirrors., 1, Aperture: The effective width (distance) of the, ‘spherical mirror from which reflection of light, can take place is called its aperture. In Figure, 4.9, it is denoted by MN. The aperture of a mirror, represents the size of the mirror., , onvex mirror ., , , , Parameter, , , , Concave mirror, , , , Convex mirror, , , , , , 1. Reflection of light, , , , kit, 4. Focus, , , , , , , , , , , , , (or bent-in surface)., , 2. Nature BOE }, . \ converges at a point in front of the mirror, after reflection., 3. Action Iti, , , , It has a real focus., , ! Reflection takes place at the concave surface, , A parallel beam of light falling on this mirror, , , , , Reflection takes place at the convex surface, + (or bulging out surface)., , , , , , , , , , | A parallel beam of light falling on this mirror, | appears to diverge from a point behind the, mirror after reflection., , , , It is a diverging mirror., , , , | Ithasa virtual focus.
