Physics Chapter 5 PDF - Light

Physics ALLEN OVERSEAS CHAPTER 5 LIGHT '...

Physics ALLEN OVERSEAS CHAPTER 5 LIGHT 'On a windless day, the smooth surface of a lake reflects light and produces stunning images of the surroundinglandscape and sky.The light falling on the flat,smooth, surfaceof lake undergoesregular reflection." THE SPOT 1 Introduction LIGHT Much of our experience of the world comesthrough light and sound. We We "see" things only when objects send out light. Every object which we need light to see objects around us. Plants use light energy to manufacture see around us would be invisible food. In fact, light from the Sun is the source of nearly all of our energy and unless light comes from enters our it, warmth. eyes, and causes messages to be sent from our eyes to our brain. The phenomenonof light, the explanation of what it is, and of why objects are visible, has taken a long time to be understood. 2 Luminous and non luminous objects Look around, in a darkened room at night. You cannot see the object in the dark.This meansthat eye alone cannot see any objects. It is only when light from an objectenters our eyes that we seethe object. Thus, light is a form of energy which excites our sense of vision [Fig. 1]. Luminous Light Non-luminous () Luminousbody :An object which emits light by itself is called a kuminous body body source of light. The Sun and the stars are natural, luminous sources of Fig.1 Luminous and non luminous light. Electric bulbs, lanterns, fluorescent tubes and candlesare artificial objects sources of light. (ü) Non-luminousbody :A body which does not emit ight of its own is THE called a non-luminous body. SPOT A non-luminous body becomes visible only when light from a luminous LIGHT body falls on it. We are able to see a non-luminous object because the Natural sources including the Sun and other stars are luminous light reflected by it enters our eyes and produces the sensation of sight. light by a contini They produce The Moon is a non-luminous body which shines, and is visible because process called 'nuclear fusion' in it reflects the light that it receives from the Sun. In other words, moon which simpler elements combine to form bigger elements. light is reflected sunlight. www.allenoverseas.com OVERSEAS 27  Class-VIl ALLEN OVERSEAS BUILDING CONCEPTS How could we see non-luminous 1 objects and luminous objects ? Explanation When the light falls on a non-luminous object, it reflects a part of the light towards us. When this light reaches our eyes, we could see an object. A (a) A steel tumbler :we cannot see luminous object emits its own light. When this light reaches our eyes, we any object like an ignited candle could see a luminous object. Thus, for us to see an object, it must reflect placed inside or emit some light that reaches our eyes. it. 3 Objects Opaque objects An object is said to be 'opaque' it does not allow ifany light to pass through it. An opaque object reflects a part of the light falling on it and remaining part is absorbed by it. A wooden box is an opaque object. (b) A plain glass tumbler : we can Transparent objects see clearly any object like an ignited candle placed inside it. An object is said to be 'transparent' it transmits most of the light falling if on it. Only a very small part of light may be absorbed or reflected from it. We can see clearly an object present behind the transparent object, e.g. a plain glass slab is a transparent object (or medium). Translucent objects An object is said to be 'translucent' if it transmits a part of the light falling on it that is not sufficient to see clearly an object present behind it. For a highly polished surface or mirror, most of the light falling on it gets (c) A tinted or milky glass tumbler reflected. For a transparent medium like glass or water, most of the light we cannot see clearly any object like ignited candle placed inside it. falling on it gets transmitted. Fig.2 Active physics 1 AcTIVE PHYSICS Take a steel tumble, a glass tumbler and a milky or tinted glass tumbler. Take three wax candles of sizes smaller than the heights of the tumblers. 2. Place one candle each inside the above tumblers.Now, ignite all thethree candles placed inside the tumblers. You will not see candle or its flame placed in the steel tumbler (see fig.2l)]. You will see the candle and its flame clearly placed in the glass tumbler (see fig.2(b)). The candle and its (a) A Convergent beam flame will appear to you blurred that was placed in the milky or tinted glass tumbler (see fig.2(). Thus,we can conclude that the steel tumbler is an opaque object, the glass tumbler is a transparent object and the milky or tinted glass tumbler is a translucent object. Some basic terms Ray of light :A ray of light is the direction in which light travels. (b) A divergent beam Beam of light :A bundle of light rays is called beam of light (or light beam). Convergentbeam:A beam of light in which all the rays move towards a single point is called convergent beam (fig.3 (a). Divergent beam :A beam of light in which all the rays emerge out from a single point is called divergent beam (fig.3 (b). (c) A parallel beam Parallel beam :A beam of light in which all the rays are parallel to each Fig.3 Various beams of light other is called parallel beam (fig.3 (c). 28 OVERSEAS www.allenoverseas.com  Physics ALLEN OVERSEAS Rectilinear propagation of light THE On a misty night, you may have seen the beams of light from the headlamps of a car, or you may beams of light have had fun watching the powerful SPOT rotated in the sky by circus companies or the organisers of a fair. You may LIGHT even have seen beams of sunlight making their way through the clouds or Human-made light sources, as through the leaves of trees. Perhaps you have noticedthat all such beams candles and lamps, rely on chemicol of light are always straight. Light always travels in straight lines. This is reactions or electricity to produce called the rectilinear propagation of light. light. There are three main effects of rectilinear propagation of ight 1. Formation of shadows 2. Formation of eclipse 3. Formation of images Properties of light Pin hole (1) Light isa form of energy that travels in the fom of waves. Screen (2) Light waves spread out in all directions as they move away from a source. (3) Light can travel through empty space (vacuum),withoutneeding a solid, liquid, or gas medium. (4)Light rays from any source always travel in straight lines. This is called rectilinear propagation of light. (5) Regardless of its source, a ray of light will not change direction unless it travels through a different medium or is disturbed in some way. (6) A light wave will spread out if it travels past the edge of a object or it if A light source movesthrough a narOw opening (seefig.4). (7) Light travels through space at the fastest speed, about 300000 km/s Fig.4 Light waves spread out in all or 3 x10 m/s. Thespeed of light is represented in scientific formulas directions if they pass through a very small hole. by the letter 'c'. In other transparent medium like water or glass, the speed of light is slightly less than the speed of light in space. (8) Light has colours.Ordinary light that we see in everyday life is called white light as it appears white to us. This white light consists of seven colours namely violet, indigo, blue, green, yellow, orange, red. (9) Light has different intensities i.e., it can be bright or dim. For example, THE a 100 W bulb will glow brighter than a 40 W bulb. Sunlight in earty SPOT 1. moming is dimmer than AcTIVE PHYSICS Take three 2 rectangular the sunlight cardboard sheets of equal in the afternoon. size and make a tiny Illumination is LIGHT the intensity of light falling on a surface, how bright i.e., hole using a pin in the centre of each. The centre of the cardboard can be is the light falling on a surface located by drawing the diagonals of the rectangularsheets. The point of (illuminance). intersection of the diagonals is the centre of the cardboard sheet. 2 Now, fix each cardboard sheet in vertical position by simply pasting them on a woodenor cardboard base using an adhesive (glue) so that their centres are in the same horizontal line (see fig.5(a)]. Let us mark these sheets as A, B and C. 3. Place a burming candle in front of the sheet A and look through the pin hole in sheet C. You wll see the candle clearly. THE 4 Now remove the sheet B from its position and paste it again at small SPOT distance away from its original position [seefig.5(b)]. Again look through LIGHT pin hole C. This time you willnot see the candle. Light is an electromagnetic wave From this activity. we conclude that light travels in a straight line. When pin which does not require any medium holes on the cardboard sheets are in straight line, the light passes through for propagation. them. When pin holes on the cardboad sheet are not in straight line, light fails to pass through them. www.allenoverseas.com OVERSEAS 29  Class-Vl ALLEN OVERSEAS Candle flame THE SPOT LIGHT Observer The unit of illumination in the Sl system (a) Candle flame can be seen as all three sheets are in a straight line is metre candle or lux. Sheet B is Light is obstructed A displaced Candle at sheetE flame slightly Observer (b) Candle flame cannot be seen as all three sheets are not in a straight line Fig.5 Active physics 2 Reflectionof light rays THE As light hits an object, some of the light is reflected or bounced off the |SPOT object. The light travels back to your eye and lets you to see the object. LIGHT Reflection is thebouncing of light rays off a surface. In otherwords, 'reflection' beam of light When light falls boundary separating ona two media, surface USually a part is reflected, a part is or a is the sending back AcTIVE PHYSICS 1 Fix 3 of all or a a white sheet of paper on a drawing board or a part of a as table. it strikes a surface. Take a comb and transmitted (refracted) and a part is absorbed. close all its openings using a black paper sheet except one in the middle. Hold the comb perpendicularto the sheet of paper. 2. Mark a point O at the middle of the bottom edge of the mirror (see fig.6].Then use the protactor and the ruler to drawa line on the paper perpendicular to the mirror from the mark. Label this line N. This line is called 'normal'.Draw a line on the paper from O at a particular angle say 30°to line N. 3. Turn on the flashlight (torch) and place it sothe beam is along the 30°line. This is the angle of incidence (i). Measure and record the angle that the beam makeswith line N. This is the angle of reflection. reflected Now, measure the angle of reflection (r). You will find that the angle of reflection is 30°. Repeat this activity, by making other angles such as 45°, 60°with the line N. In each case, you find that the 'angle of incidence wil is equal to the angle of reflection'. THE SPOT Incident LIGHT Mirror ray The laws of reflection holdgood for of waves. These laws are ble to app plane as well as curved Reflected surfaces. rav Torch Table Fig.6 Active physics 3 30 OVERSEAS www.allenoverseas.com  Physics ALLEN OVERSEAS Some basic terms Incident ray :The ray of light which falls on the mirror surface is called 'incident ray'. Reflected ray :The ray of light which is sent back by the mirror is called Reflected ray 'reflected ray'. Normal Point of incidence:The point at which the incident ray falls on the Incident ray miroris called 'pointof incidence' Normal :A line perpendicular to the surface of mirror passing through the point of incidence is called 'nomal'. Angle of incidence :The angle made by incident ray with the normal at the point of incidence is called 'angle of incidence'. Surface Angle of reflection :The angle made by reflected ray with the nomal at Plane of the point of incidence is called 'angle of reflection'. travel Laws of reflection Fig.7 Reflection of ight On reflection of light from a surface, the speed, wavelength and frequency of light do not change. This is because the light stays in the same medium. But, amplitude and intensity of reflected ray is slightly less than that of incident ray as somepart of energy is absorbed at the surface. First law The incident ray,the reflected ray and the normal at the point of incidence, all lie in the same plane. Second law The angle of incidence is equal to the angle of reflection. 2i= 2r M nnnmnnInT M, BUILDING CONCEPTS Fig.8 Reflection of light ray when it How is a ray of light reflected when it is incident normally on a plane is incident normally on a plane mirror? miror. Explanation For a ray incident normally on a surface, the angle of incidence = 0°, i thereforethe angle of reflection r = 0°. Thus, a ray of light AO incident normally on a surface is reflected along the same path OA as shown in fig. 8. BUILDING CONCEPTS 3 How can you find the angle of deviation? Explanation Let an incident ray 1 falls on a plane mirror at an angle of incidence x, lane mirror then by the law of reflection, the angle of reflection is also x (see fig.9). Now, we can find angle y from the figure, x +x +y= 180° (angle of a straight line is 180) Fig.9 Finding angle of deviation or 2x + y= 180° or y= 180°- 2x Angle y is known asangleof deviation. www.allenoverseas.com OVERSEAS 31  Class-VII ALLEN OVERSEAS NUMERICAL ABILITY 1 Figure 10 shows an incident ray AO and the normal ON on a plane miror. The angle which the incident ray AO makes with the mirror is 30°. Find the angle of incidence and angle of reflection. What is the angle of deviation? 30 Solution As shown in the fiqure 10, the angle between the surface of the mirror and the incident ray is 30°. Fig.10 Numerical ability 1 i+30° =90° 2i=90°-30° = 60° 2i=60° 4=Zr (According to law ofreflection) Zr=60° Given, the angleof incidence,x =60°. The angle of deviationis given by. y- 180°- 2x y= 180°-2 x60° V= 180- 120°= 60° CHECK YoUR CONCEPTS 1 1. Do we see all objects due to reflected light ? 2. What are the uses of transparent, translucent and opaque materials? 3. Is possible to have a ray of light ? it 6 Regularand diffuse reflection Regular reflection (specular reflection) Ifa parallel beam of light is incident on a smooth plane miror, the reflected beam is also a parallel beam. This reflection is called regular reflection'. Reason incidence.This : The light is rays strike the smooth surface at same angles of because all the nomals are parallel to each other on the smooth surface. Thus, the light rays get reflected at sameanglesof reflection. Smooth surface That is why, the rays of reflected beam remain parallel to each other (a) Regular reflection Ifig.11 (a)). Diffuse reflection (irregularreflection) If a parallel beam of light is incident on a rough surface or mirror, the rays of light become non-parallel to each other. Such a reflection is called 'diffuse reflection'. Reason :The light rays strike the rough surface at many different angles of incidence. This is because all the normals are not parallel to each other on the rough surface.Thus, the light rays get reflected at many different angles.That is why, the rays of reflected beam become non-parallel to Rough surface each other [fig.11 (b). BUILDING CONCEPTS Why do you 4 seeyour image in water when you stand on the edge of a lake and look in the water ?Is it always possible to see a cdear image in water? (b) Diffuse reflection Explanation Fig. 11 Reflection through You see your image in water due to reflection of light from the surfaceof smooth and rough surfaces water. When light bounces off a smoothsurfaceof lake, the reflection is clear. You can see exactly what is reflected. But if a strong wind blew across the smooth lake, it would be hard to see your reflection clearly in the way water. The light would be bouncing off in all directions, and your reflection would be blurry (unclear). 32 OVERSEAS www.allenoverseas.com  Physics ALLEN oVERSEAS 7 Plane mirror When you look into a plane mirror, you see a clear image of yourself. The Object Image imageis formed due to reflection. Reflectionalways involves two rays - an incoming or "incident" ray and an outgoing or "'reflected" ray. When an object is viewed in a mirror, the eyes take in the light that has been reflected. But the brain assumes that the light rays have reached the eyes in straight lines. The brain works backwards along the light paths and perceives an image behind the mirror. Fig. 12 Formation of Reflection from plane mirrors image by plane mirror A mirror is a highly polished surface used to reflect the light falling on it. Mirrors areusually made by depositing a thin layer of silver metal on one side of a plane glass sheet. Some basic terms Object :Anything which gives out light rays either of its own or due to reflection is called an 'object'. Point object :An object whose dimensions are negligibly small is called 'point object' [Fig. 12). Extended object :An object whose dimensions are quite large is called 'extended object' [fig. 13]. Point Objects Symbols used in Extended Objects ray diagrams Fig. 13 Reflection from plane miror Image:An image of an object is formed when light rays coming from the object meet or appear to meet at a point after reflection from a mirror or refraction from a lens. Object Image Real image :A real image is one which is formed when the light rays A actually meet at a point and which can be obtained on a screen. Virtual image :A virtual image is one which is formed when the rays do not actually meet at a point but they appearto meet at a point. Such B B images can not be obtained on the screen. Image formed by a plane mirror The properties of image formed by plane mirors are (see fig. 14) (1) The image is virtual and erect. (2) The distance of imagefrom mirror is equal to distance of object from Fig. 14 Fomation of miror. image of an extended object (3) The size of image is exactly equal to the size of object. (4) The image is laterally inverted. If the object is shifted by the distance d' towards mirror, the image will also shift by the same distance 'd' towards the mirror. If a person moveswith a speed 'v' towards (or away) from the mirror, the imageto him will appear to move with a speed '2v'towards (oraway) from him. Lateral inversion When an asymmetric object is placed in front of a plane miror, then the ABC right side of the object appears to be the left side of image and the left side of the object appears to be the right side of its image. This change of sides of an object seen in the image is called 'left - right inversion' or 'lateral inversion (see fig. 15). The image is inverted side ways, thus, also called Fig.15 Lateral inversion of image in a plane miror 'side ways inversion' www.allenoverseas.com OVERSEAS 33  Class-VIl ALLEN OVERSEAS BUILDING CONCEPTS 5 Window glass When looking outdoors through a glass window at night, why do we sometimes see a double image of ourself ? Explanation Incident Reflection occurs whenever a ray of light strikes a surface separating two Ray different media. For the glass in a window,there are twO such surfaces. The first is the inner surface of the glass,and the second is the outer surface [see fig. 16]. Each of these interfaces results in an image. Thus, we see a double imageof ourselves. Two reflected rays 2 CHECK YoUR ANSWERS 1. Almost everything 1 we see around us, can be seen due to reflected light. Inner Outer surtace surface Moon, for example, receives light from the Sunand reflects it, that's why we see the Moon. 2. Transparent materials are used in making window panes, spectacles, Fig.16 Building concepts 5 binoculars, telescopes etc. Windows and doors are made of translucent materials so that only a small amountof light can enter the room. Opaque objectsare used when we do not want to allow light to pass through it. 3. Aray of light is an idealization. In reality, we have a narrow beam of light which is made up of several rays. For simplicity, we use the term ray for narrow beam. BUILDING CONCEPTS 6 You may have observed the imageof the sun in the windows of distant buildings near the time that the sun is rising or setting. However, the imageof the sun is not seen in the windowsof distant building during midday. Explain it, by drawing appropriate light rays on the diagram. Explanation A ray light (Ray 1) drawn from the sun's position of at 6.30 pm to the distantwindow gets reflected from the window and travels to the observer's eye (see fig. 17]. While a ray of light (Ray 2) drawn from the 12.30pm sun positionto the windowwill reflect and travel to the ground, thus does not reach to the distant observer's eye. Sun THE 12.30 pmy SPOT Ray 2 Sur LIGHT 6.30 Ray 1 House The word 'AMBULANCE is written from right to left (as 3MAJUSMA). Windows Since in the mirro, the image is lateraly inverted so the driver of the Observer 2 vehicde aheadcan reod "AMBULANCE" written on Fig. 17 Building concepts 6 it. CHECK YoUR CONCEPTS 1. Why does wet surface glow more than a dry surface ? 2. Which is easier, to read from rough pages or to read from glossy (shinny) pages ? Explain your answer. 3. Find out the letters of English alphabet in which the imagefomed in a plane mirror appear exactly like the letter itself. 4. How is a mirror made ? 34 OVERSEAS www.allenoverseas.com  P ALLEN OVERSEAS BUILDING CONCEPTS Man A is standing, 7 and facing a plane mirror while man B is running ms. How fast doesthe image towards him from behind at a speed of 1 of man B seem to be running as observed by (a) man A (b) man B ? Mirror +-1 ms Man A man B Fig. 18 Building concepts 7 Explanation In the above figure, man A isstanding facing a plane miror while man B is nunning towards the samemiror (coming from behind man A)at speed 1 m/s. ) Following conclusions can bedrawn in this event : If man A observes in the mirror, the image of man B in the miror seems to moves towards him at speed 1 m/s. (i) Man B is running towards mirror at speed I m/s.Also, his image in the mirror is runing towardshim at speed 1 m/s.Since the direction of motion of both 'man B'and his image are opposite to each other, the speed of image of man B' as seen by the 'unning man will be B 1 m/s + l m/s =2m/s. 1 ms Mirror Image of Image of l ms' Man B Man A Man A Man B Fig. 19 Bilding concepts 7