LIGHT - ncert.nic.in

1 LIGHTLIGHTThe world is largely known throughthe senses. The sense of sightis one of the most importantsenses. Through it we see mountains,rivers, trees, plants, chairs, people andso many other things around us. We alsosee clouds, rainbows and birds flyingin the sky. At night we see the moonand the stars. You are able to see thewords and sentences printed on thispage. How is seeing made possible? What makes ThingsVisibleHave you ever thought how we seevarious objects? You may say that eyessee the objects. But, can you see anobject in the dark? It means that eyesalone cannot see any object. It is onlywhen LIGHT from an object enters oureyes that we see the object. The lightmay have been emitted by the object, ormay have been reflected by learnt in Class VII that a polishedor a shiny surface can act as a mirror.

2 Amirror changes the direction of lightthat falls on it. Can you tell in whichdirection the LIGHT falling on a surfacewill be reflected? Let us find Laws of ReflectionActivity a white sheet of paper on adrawing board or a table. Take acomb and close all its openingsexcept one in the middle. You canuse a strip of black paper forthis purpose. Hold the combperpendicular to the sheet of LIGHT from a torch throughthe opening of the comb from oneside (Fig. ). With slightadjustment of the torch and thecomb you will see a ray of LIGHT alongthe paper on the other side of thecomb. Keep the comb and the torchsteady. Place a strip of plane mirrorin the path of the LIGHT ray ( ). What do you observe?After striking the mirror, the ray oflight is reflected in another LIGHT ray, which strikes any surface,is called the incident ray.

3 The ray thatcomes back from the surface afterreflection is known as the reflected : Arrangement for showing reflection2015-16 (11-11-2014)SCIENCE200A ray of LIGHT is an idealisation. Inreality, we have a narrow beam oflight which is made up of severalrays. For simplicity, we use the termray for a narrow beam of lines showing the position ofthe plane mirror, the incident ray andthe reflected ray on the paper with thehelp of your friends. Remove the mirrorand the comb. Draw a line making anangle of 90 to the line representing themirror at the point where the incidentray strikes the mirror. This line is knownas the normal to the reflecting surfaceat that point (Fig. ). The angleTable :Angles of Incidenceand ofAngle ( i) Reflection ( r) you see any relation between theangle of incidence and the angle ofreflection.

4 Are they approximately equal?If the experiment is carried out carefully,it is seen that the angle of incidence isalways equal to the angle of is known as the law of us perform another activity would happen if Ithrew the LIGHT on themirror along the Activity again. Thistime use a sheet of stiff paper or achart paper. Let the sheet project alittle beyond the edge of the Table(Fig. ). Cut the projectingportion of the sheet in the at the reflected ray. Make surethat the reflected ray extends to theprojected portion of the paper. Bendthat part of the projected portion onwhich the reflected ray falls. Canyou still see the reflected ray? Bringthe paper back to the originalFig. : Drawing the normalFig. :Angle of incidence and angle ofreflectionReflectedrayNormalIncidentra ybetween the normal and incident ray iscalled the angle of incidence ( i).

5 Theangle between the normal and thereflected ray is known as the angle ofreflection ( r) (Fig. ). Measure theangle of incidence and the angle ofreflection. Repeat the activity severaltimes by changing the angle ofincidence. Enter the data in Table (11-11-2014)LIGHT201position. Can you see the reflectedray again? What do you infer?with the sun as the source of LIGHT insteadof a torch. You, too, can use the sun asthe source of activities can also be performedby making use of the Ray StreakApparatus (available in the kitprepared by NCERT).Boojho remembered that in Class VII,he had studied some features of theimage of an object formed by a planemirror. Paheli asked him to recall thosefeatures:(i)Was the image erect or upsidedown?(ii)Was it of the same size as theobject?(iii)Did the image appear at the samedistance behind the mirror as theobject was in front of it?

6 (iv)Could it be obtained on a screen?Let us understand a little more aboutthe formation of an image by a planemirror in the following way:Activity source of LIGHT O is placed in frontof a plane mirror PQ. Two rays OAand OC are incident on it (Fig. ).Can you find out the direction ofthe reflected rays?Draw normals to the surface of themirror PQ, at the points A and draw the reflected rays at thepoints A and C. How would youdraw these rays? Call the reflectedrays AB and CD, them further. Do they meet?Extend them backwards. Do theymeet now? If they meet, mark thispoint as I. For a viewer s eye at E(Fig. ), do the reflected rays(b)Fig. (a), (b) :Incident ray, reflected rayand the normal at the pointof incidence lie in the sameplaneWhen the whole sheet of paper isspread on the table, it represents oneplane.

7 The incident ray, the normal atthe point of incidence and the reflectedray are all in this plane. When you bendthe paper you create a plane differentfrom the plane in which the incidentray and the normal lie. Then you do notsee the reflected ray. What does itindicate? It indicates that the incidentray, the normal at the point ofincidence and the reflected ray all liein the same plane. This is another lawof and Boojho performed theabove activities outside the classroom(a)2015-16 (11-11-2014)SCIENCE202appear to come from the point the reflected rays do notactually meet at I, but only appearto do so, we say that a virtual imageof the point O is formed at I. As youhave learnt already in Class VII,such an image cannot be obtainedon a may recall that in an imageformed by a mirror the left of the objectappears on the right and the rightappears on the left.

8 This is known aslateral and DiffusedReflectionActivity that parallel rays areincident on an irregular surface asshown in Fig. Remember thatthe laws of reflection are valid ateach point of the surface. Use theselaws to construct reflected rays atvarious points. Are they parallel toone another? You will find thatthese rays are reflected in differentdirections. (Fig. )When all the parallel rays reflectedfrom a plane surface are not parallel,the reflection is known as diffused orirregular reflection. Remember that thediffused reflection is not due to thefailure of the laws of reflection. It iscaused by the irregularities in thereflecting surface, like that of the other hand, reflection froma smooth surface like that of a mirroris called regular reflection (Fig. ).Images are formed by : Image formation in a plane mirrorFig.

9 :Parallel rays incident onan irregular surfaceFig. :Rays reflected from irregularsurfaceFig. : Regular reflection2015-16 (11-11-2014)LIGHT203 Let us find LIGHT Can beReflected AgainRecall the last time you visited a hairdresser. She/he makes you sit in front ofa mirror. After your hair cut is complete,she/he holds a mirror behind you to showyou how the hair has been cut(Fig. ). Do you know how you couldsee the hair at the back of your head?Paheli recalls having constructed aperiscope as an Extended Activity inClass VI. The periscope makes use oftwo plane mirrors. Can you explainhow reflection from the two mirrorsenables you to see objects which arenot visible directly? Periscopes areused in submarines, tanks and alsoby soldiers in bunkers to see Multiple ImagesYou are aware that a plane mirror formsonly a single image of an object.

10 Whathappens if two plane mirrors are usedin combination? Let us We See all Objects due to Reflected LIGHT ?Nearly everything you see around is seen due to reflected LIGHT . Moon, for example,receives LIGHT from the sun and reflects it. That s how we see the moon. The objectswhich shine in the LIGHT of other objects are called illuminated objects. Can youname some other such objects?There are other objects, which give their own LIGHT , such as the sun, fire, flameof a candle and an electric lamp. Their LIGHT falls on our eyes. That is how we seethem. The objects which emit their own LIGHT are known as luminous : Mirror at the hair dresser shopI have a question. Can thereflected rays be further reflectedif incident on another mirror?2015-16 (11-11-2014)SCIENCE204 Activity two plane mirrors.