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CHAPTER, , , , Light, , ight is a form of energy. Light is needed to see things around us. We are able to see the beautiful, , world around us because of light. We can read a book, see picture ina magazine and watch TV and, , movies due to the existence of light. And it is light which makes us see our image in a looking, mirror. We detect light with our eyes., , What Makes Things Visible, , Though we see various things (or objects) around us with our eyes but eyes alone cannot see any, object. For example, we cannot see objects in a dark room or in the darkness of night even when our eyes, a source of light to make the objects (or things) visible. So, as soon as light from a, torch (an electric bulb or a tube-light) falls on the object, we are able to see it clearly even ina dark room, or in the darkness of night. If is only when light coming froman object enters our eyes that we see that object. This, mitted by the object itself or may have been reflected by the object. Thus, it is light, sible to us. Light enables us to see things from which it comes or from which it, , are wide open. We need, , , , light may have been e, which makes things vi, is reflected., , Luminous Objects and Non-Luminous Objects, There are two types of objects around us : luminous objects and non-luminous objects. The objects, , which emit their own light are called luminous objects. The luminous objects are, in fact, tHe sources of, light, Luminous objects produce their own light and then emit this light. The sun, other stars, lighted, , electric bulb, glowing tube-light, torch, fire, and flame of a burning candle, are all luminous objects. A, luminous object can be seen because the light given out by it enters our eyes. For example, we can see the, Sun because the light given out by sun (or light emitted by sun) enters our eyes. Luminous objects are very, small in number., , All the objects cannot give out th, called non-luminous objects. Actually,, luminous objects cannot produce light, therefore,, planets, table, chair, book, trees, plants, flowers,, and roads, are some of the examples of non-lumi, , eir own light. The objects which do not emit their own light are, the non-luminous objects cannot make their own light. Since nonthey cannot emit their own light. The moon, earth, other, human beings, fan, bed, mirror, diamond, walls, floor, nous objects. In fact, most of Hie objects around us are nont, 293

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(294) @ AWARENESS SCIENCE FOR EIGHTH CLASS I ununous objects (which do not have light of their own). The non-luminous objects can be seen only whey, light coming from a luminous object falls on them. This light is reflected by the non-luminouso ject in a, directions. And when this reflected light enters our eyes, we can see the non-luminous object. This ;,, because to us the light appears to be coming from the non-luminous object. Thus, we can see the non., luminous objects because they reflect light (received from a luminous object) into‘our eyes.For example,, the moon is a non-luminous object which does not have its own light. We can see the a Because Toon, reflects light (received from the sun) into our eyes. Thus, moon is a reflector of sunlight. Sinulatly, We can, see this book because the sunlight (bulb-light or tube-light) falling on it is reflected by the shook into our, eyes. Thus, the non-luminous objects shine in the light of luminous objects and become visible to us. The, , non-luminous objects are also called illuminated objects (because they get a ‘“ ee, , the light of luminous objects falling on them). Most of the objects around us (being mon-luminous) are seen, by the reflected light., , Reflection of Light, , When light falls on the surface of an object, the object sends this light back. The process of sendin,, back light rays which fall on the surface of an object, is called reflection of light. The reflection of light, is studied by using a plane mirror. A plane mirror reflects almost all the light which falls on it. This means, that a plane mirror changes the direction of light which falls on it. We will now study the direction in which, the light falling on a plane mirror is reflected., , In order to study the reflection of light, we need an apparatus which can produce a thin beam of light., We use an apparatus called ‘ray-box’ to produce a thin beam of light in science activities. A ray-box has a, light bulb inside it and there is a narrow slit in front of the box (see Figure 1). When the light bulb is, switched on, a very thin beam of light (or a narrow beam of light) comes out of the narrow slit of the raybox. This narrow beam of light is then used to study the reflection of light from a plane mirror. Thus, the, ray-box acts as a source of light in the ‘reflection of light’ activities. The thin beam of light produced by, a ray-box is visible on a white sheet of paper, so its path on paper can be traced by using a pencil. Please, note that though a thin beam of light is made up of several rays of light but for the sake of simplicity and, , convenience, a thin beam of light is considered to be a ray of light. The ‘ray-box’ is also known as ‘raystreak apparatus’., , i ACTIVITY TO STUDY THE REFLECTION OF LIGHT FROM A PLANE MIRROR, , We take a plane mirror strip MM’ and place it sideways on a white sheet of paper so that its reflecting, surface (shining surface) is towards the left side (see Figure 1). Mark the position of mirror on the sheet of, paper with a pencil., , |, \l, , 7 Narrow, , ao Vin |, Plane =, ve en mirror, Zz, L\\ iy, Ray box —” ene, (Source of light) (oe ry, , N-----------5----- 2-5-2, , Normal en oa, , , , , Point of |, incidence ., , , , w, Figure 1. Arrangement to study the reflection of light from a plane mirror,

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LIGHT # [295 ), , Keep the ray-box at posivon A in front of the plane mirror (see Figure 1). By opening the slit of ray-box,, | shine ¢ narrow beam of light AO on the plane mirror. We will see that the beam of light AO strikes the mirror, | uriace at point oO, it gets reflected and then goes in another direction OB (see Figure 1). We mark the point, | gon the sheet of paper and also trace the paths of rays of light AO and OB on the sheet of paper by using, | scale and pencil. At point O we draw a line ON perpendicular to the surface of the mirror MM”., , Let us Measure the angles AON and NOB. We will find that the angle AON is equal to the angle NOB., | Now, the angle AON is the angle of incidence and the angle NOB is the angle of reflection, so this activity, sows that the angle of reflection is equal to the angle of incidence. In this activity, the incident ray AO, the, , reilected ray OB and the normal ON, all lie in the plane of paper. They neither come up out of paper nor, , go down into paper. This shows that the incident ray, the reflected ray, and the normal (perpendicular) at the, tof incidence, all lie in the same plane., , , , poi, , , , Before we go further and study the laws of reflection of light, it is necessary to understand some, important terms connected with the reflection of light clearly. These terms are : Incident ray, Point of, incidence, Reflected ray, Normal, Angle of incidence and Angle of reflection. These are described below :, , 1. The ray of light which falls on the mirror surface is called incident ray. In Figure 1, the ray of light, AO coming from the ray-box falls on the mirror surface, therefore, AO is the incident ray. The incident ray, tells us the direction in which the light from a source falls on the mirror. The incident ray always goes, towards the mirror., , 2, The point at which the incident ray strikes the mirror is called the point of incidence. In Figure 1,, the incident ray AO strikes the mirror (or touches the mirror) at point O, therefore, O is the point of, incidence. The point of incidence tells us where exactly light falls on the mirror surface., , 3. When the incident ray falls on a mirror, the mirror sends it back in another direction. And we say, that the mirror has reflected the ray of light. The ray of light which is sent back by the mirror is called, the reflected ray. In Figure 1, the ray of light OB is sent back by the mirror, therefore, OB is the reflected’, ray. The reflected ray tells us the direction in which the light goes after reflection from the mirror. The, reflected ray always goes away from the mirror. Please note that there can be only one reflected ray for a given, single incident ray falling on a plane mirror. This is because the same ray of light is called incident ray before, it strikes the mirror and becomes reflected ray after it rebounds from the mirror., , 4. The ‘normal’ is a line drawn at right angles to the mirror surface at the point of incidence. In other, words, the ‘normal’ is a line which is perpendicular to the mirror surface at the point of incidence. In, Figure 1, the dotted line ON is the normal to the mirror surface MM‘ at the point of incidence O. We usually, represent ‘normal’ to the mirror by a dotted line to distinguish it from the incident ray and the reflected ray., Please note that ‘normal’ is just a line which is perpendicular to the mirror surface, and it should not be, called ‘normal ray’. The ‘normal’ is an imaginary line which is drawn on paper for the sake of convenience, in understanding, the laws of reflection. Please note that the ‘normal’ lies exactly in-between the incident, ray and the reflected ray, 5. The angle between incident ray and normal is called the angle of incidence. In Figure 1, AO is the, incident ray and NO is the normal. So, the angle AON is the angle of incidence. The angle of incide, tepresented by the letter i i= incidence). Please note that the angle of incidence is made by the, With the normal to the mirror surface and not with the mirror surface itself,, , 6. The angle between reflected ray and normal is called the angle of reflection. In Figure |, Teflected ray is OB and the Ee NO. So, the angle BON is the angle of reflection (We gure 1, the, the angle NOB is the angle of reflection). The angle of reflection is represent, , Laws of Reflection of Light |, When a ray of jight falls on a plane mirror, it gets refle, a plane mirror takes place according to two laws which, , of reflection of light are as follows :, , Nee is, incident ray, , can also say that, by the letter (7 = reflection), , cted (see Figure 2). The retle, , aielknowneas' ction of light from, , aWws of reflection of light. The laws

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{ 296) @ AWARENESS SCIENCE FOR EIGHTH CLASS, , Plane M, , ; A According to the first law of reflection: The A, incident ray, the reflected ray, and the normal (at the, point of incidence), all lie in the same plane. In, Figure 2, the incident ray AO, the reflected ray OB and, the normal ON, all lie in the same plane, the plane of, paper. They are neither coming up out of the paper; N, nor going down into the paper., , , , , Angle of, Normal incidence, , , , , , , incidence, 2. According to the second law of reflection : The, angle of reflection is always equal to the angle of eel, incidence. If the angle of incidence is i and the angle of B _, reflection is r, then : / ", . Figure 2., 4i= Zr, In Figure 2, if we measure the angle of reflection NOB, we will find that it is exactly equal to the angle, of incidence AON. A Plane, The second law of reflection will become more A filang mitor, , , , clear from the following examples : The second law of me, , reflection says that the angle of reflection is always, equal to the angle of incidence. This means that if the, angle of incidence for a ray of light is 35°, then the angle, of reflection will also be 35° (because they have to be, , equal). This is shown in Figure 3. In Figure 3, the angle je Ww pee te, of incidence AON is 35°, so the angle of reflection NOB BE ff, , is also 35°., , If we change the angle of incidence, the angle of Figure 3. Riguress., , reflection will also change accordingly. The new angle of reflection will also be equal to the new angle of, incidence. For example, if the angle of incidence is changed to 45°, then the angle of reflection will also, change and become 45° (see Figure 4). Now, when the angle of incidence is 45°, then the angle of reflection, is also 45°. So, in this case the reflected ray is at an angle of 45° + 45° = 90° to the incident ray. From this, we conclude that if the reflected ray is at an angle of 90° to the incident ray, then the angle of incidence will, , 0°, , be half of 90°, that is, as 45° (see Figure 4) ., , We will now describe what happens when a ray of light falls normally (or, , , , , , . * ey: ae Plane M, perpendicularly) on the surface of a plane mirror. When a ray of light is incident tniror’”, normally (or perpendicularly) on a plane mirror, it means that it is travelling leiden t, along the ‘normal’ to the mirror surface (see Figure 5). The angle of incidence for ray, , Reflected, ray, , such a ray of light is zero. Since the angle of incidence is zero, so according to the “ “(Normal), second law of reflection, the angle of reflection should also be zero. This means, , that the reflected ray will also travel back from the mirror along the normal (see t, Figure 5). Thus, a ray of light which is incident normally (or perpendicularly) - Ww, on a mirror is reflected back along the same path. This is because the angle of Figure 5., incidence for such a ray of light is 0° and the angle of reflection is also 0°. Thus, if the incident ray goes to, a mirror along normal, the reflected ray will also travel back along normal. In this case the same line, represents incident ray, normal and the reflected ray (see Figure 5)., , Please note that whenever light is reflected, laws of reflection are obeyed. As we will see after @, while, we can find out the nature and position of an image formed by a plane mirror by using the laws of, reflection of light. We will now answer some questions based on the laws of reflection of light, , Sample Problem 1. An incident ray makes an angle of 35° with the surface of a plane sR What is, the angle of reflection ?

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LIGHT = | 297, , : gle of r i ., * of incidence. In this case, the ine “flection, we should first know the a ior ig, , ident, 18" of the mirror (see Figure 6), so t nt ray makes an angle of 35° with the, , , , , , , afi ice is the angle between ae angle of incidence is not 35°. The angle \, , fil + incidence will be 90° — 35°. gee MLE Tay and normal. So, in this case, the Angle of, , angle? sevanclesotivenecs = 55 - Since the angle of incidence is 55 degrees, a, , ipeefore? 8 ensction is also 55 degrees. This is shown clearly in. “77777, , figure 6 * A Oe, , a PTR toeoroi reflection, , i as shown in Hi os 0 Mirrors PQ and QR are kept at right angles to, , ach ° £30° as Sesh 5 A ray of light AB is incident on the mirror PQat, , an ang en Gh anda In Figure 7. Draw the path of the reflected ray from the, , gecond mirror Q® ani ind the angle of reflection for the mirror QR Figures, , . . (NCERT Book Question), Solution. (i) When the ray of light AB is incident on plane mirror PQ, making an angle of incidence ABN of 30°, it will be reflected from the mirror, pQ making an equal angle of reflection of 30° with the normal BN. So, we, draw a line BC making an angle of 30° with the normal BN (see Figure 8). The, , line BC will be reflected ray of light and the angle NBC (of 30°) will be the, angle of reflection for the mirror PQ (see Figure 8)., , , , (ii) The reflected ray BC of mirror PQ meets the second mirror QR at, point C making an angle of 30° with the surface of mirror QR (This is, because angle NBC and angle BCQ are alternate angles and hence equal) (see, Figure 8). The reflected ray BC of mirror PQ becomes incident ray BC for the, mirror QR. The angle of incidence for ray BC on mirror QR will be 90° — 30°, = 60°. Since the angle of incidence for ray of light BC on mirror QR is 60°,, therefore, the reflected ray CD for mirror QR will also make an equal angle, of reflection of 60° (as shown in Figure 8)., , REGULAR REFLECTION AND DIFFUSE REFLECTION OF LIGHT, , , , Figure 8, , , , In regular reflection, a parallel beam of incident light is reflected as a parallel beam in one, direction. In this case, parallel incident rays remain parallel even after reflection and go only in one, direction (see Figure 9). Regular reflection of light occurs from smooth surfaces like that of a plane, mirror (or highly polished metal surfaces). For example, when a parallel beam of light falls on the smooth, surface of a plane mirror, it is reflected as a parallel beam in only one direction as shown in Figure 9, Thus,, a plane mirror produces regular reflection of light. Images are formed by regular reflection of light. For, example, a smooth surface (like that of a plane mirror) produces a clear image of an object due to regular, reflection of light. A highly polished metal surface and a still water surface also produce regular reflection, of light and form images. This is why we can see our face in a polished metal object as well as in the still, water surface of a pond or lake. A polished wooden table and a marble floor with water spread over it are, very smooth and hence produce regular reflection of light., , : The regular reflection of light from a smooth surface can be explained as follows : All the particles, of a smooth surface (like a plane mirror) are facing in one direction. Due to this the angle of incidence for, all the parallel rays of light falling on a smooth surface is the same and hence the angle of reflection for all, the rays of light is also the same . Since the angle of incidence and the angle of reflection are the same (or equal), a beam of parallel rays of light falling on a smooth surface is reflected as.a beam of parallel light rays in one direction, only (see Figure 9)., , fy