05. REFRACTION OF LIGHT AT PLANE SURFACES

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1 CLASS-10 PHYSICAL SCIENCE 05. REFRACTION OF LIGHT AT PLANE SURFACES Questons and Answes 1. Why s t dffcult to shoot a fsh swmmng n wate? Due to efacton at wate and a nteface, the fsh appeas to be ased and seems to be close to the suface. The s called appaent depth. The shoote ams the gun to appaent poston of fsh nstead of eal poston. Hence t s vey dffcult to shoot a fsh swmmng n wate. 2. The speed of the lght n a damond s 1, 24, 000 km/s. Fnd the efactve ndex of damond f the speed of lght n a s 3,00,000 km/s. nagamuthy.weebly.com Speed of lght n a (v 1 ) =300000km/s Speed of lght n damond (v 2 ) =124000km/s Refactve ndex of damond = = = Refactve ndex of glass elatve to wate s 9/8. What s the efactve ndex of wate elatve to glass? Refactve ndex of glass elatve to wate s n gw = = Refactve ndex of wate elatve to glass s n wg = = 4. The absolute efactve ndex of wate s 4/3. What s the ctcal angle? Absolute efactve ndex of wate (n) = Ctcal angle (c) =? Sn C = Sn C = = 0.75 = Sn 48o 36 I C = 48 o 36 I 5. Detemne the efactve ndex of benzene f the ctcal angle s Refactve ndex of Benzene (n) =? Sn C = n = Ctcal angle (c) = 42 o = =. = Explan the fomaton of mage? Mage s an optcal lluson whee t appeas that wate has collected on the oad at a dstant place but when we get thee, we don t fnd any wate. The fomaton of a mage s the best example whee efactve ndex of a medum vaes thoughout the medum. Dung a hot summe day, a just above the oad suface s vey hot, acts as eae medum. And the a at hghe alttudes s cool,acts as dense medum. Thus the efactve ndex of the coole a at the top s geate than the efactve ndex of hotte a just above the oad. When the lght fom a tall object such as tee passes though a medum just above the oad, whose efactve ndex deceases towads gound, t suffes, efacton and takes a cuved path because of total ntenal eflecton. Hence we feel the lluson of wate beng pesent on oad whch s the vtual mage (mage) and an nveted mage of tee on the oad. 7. How do you vefy expementally that sn /sn s a constant? Pocedue: Take a wooden plank. Cove wth whte chat. Daw two pependcula lnes, passng though the mddle of the pape. Mak one lne as NN whch s nomal to the anothe lne maked as MM. Hee MM epesents the lne dawn along the nteface of two meda and NN epesents the nomal dawn to ths lne at O.

2 Take a potacto and place t along NN (ts cente concdes wth O). Then mak the angles fom 0 o to 90 o on both sdes of the lne NN. Repeat the same on the othe sde of the lne NN. Now place a sem-ccula glass dsc so that ts damete concdes wth the nteface lne (MM) and ts cente concdes wth the pont O. Pont a lase lght along NN n such a way that the lght popagates fom a to glass though the nteface at pont O and obseve the path of lase lght comng fom othe sde of dsc.. Send Lase lght along a lne whch makes15 o () wth NN and see that t passes though pont O. Measue ts coespondng angle of efacton (). Note these values n table. Fnd sn, sn and also the ato Sn Sn. Do the same expement fo the angles of ncdence such as 20 o,25 o, 30 o,and 35 o. In each and evey case, we get the ato as a constant. Sl. No. () () Sn Sn Sn Sn 1 15 o 2 20 o 3 25 o 4 30 o 5 35 o 8. Explan the phenomenon of total ntenal eflecton wth one o two actvtes. Take a cylndcal tanspaent vessel of 1 lt. Place a con at the bottom of the vessel. Now pou wate untl you get the mage of the con on the wate suface (look at the suface of wate fom a sde). Ths s the phenomenon of total ntenal eflecton. One of that s a mage whch we wtness whle dvng o whle walkng on a oad dung a hot summe day. If lght ay passes fom dense medum to ae medum then the efactve angle s moe than the ncdent angle. The ncdent angle fo whch the angle of efacton s 90 o, s called ctcal angle. If the angle of ncdence s moe than ctcal angle, then total ntenal eflecton occus. 9. How do you vefy expementally that the angle of efacton s moe than angle of ncdence when lght ays tavel fom dense to ae medum. Take a ccula metal dsc. Cove wth whte chat. Mak one lne as NN whch s nomal to the anothe lne maked as MM. Hee MM epesents the lne dawn along the nteface of two meda and NN epesents the nomal dawn to ths lne at O. Take a potacto and place t along NN (ts cente concdes wth O). Then mak the angles fom 0 o to 90 o on both sdes of the lne NN. Repeat the same on the othe sde of the lne NN. Aange two staws at the cente of the dsc n such a way that they can be otated feely about the cente of the dsc. Adjust one of the staws to make an angle 10 0 wth the nomal NN ( angle of ncdence ). Immese half of the dsc vetcally nto the wate, flled n a tanspaent vessel. (MM concdes the suface of the wate) Fom the top of the vessel ty to vew the staw whch s nsde the wate. Then adjust the othe staw whch s outsde the wate untl both staws appea to be n a sngle staght lne. Then take the dsc out of the wate and obseve the two staws on t. We fnd that they ae not n a sngle staght lne. nagamuthy.weebly.com

3 Measue the angle between the nomal and second staw. (angle of efacton). Note down the angle of ncdence and angle of efacton n the table. Do the same fo vaous angles lke 15 o, 20 o, 25 o, 30 o, 35 o and 40 o. Fnd the coespondng angles of efacton and note them. Sl. No. () () 1 10 o 2 15 o 3 20 o 4 25 o 5 30 o 6 35 o 7 40 o We obseved that the angle of efacton() s always geate than the angle of ncdence (). 10. Take a bght metal ball and make t black wth soot n a candle flame. Immese t n wate. How does t appea and why? (Make hypothess and do the above expement). Take a metal ball and make t black wth soot n a candle flame. Immese the ball n a wate beake. A thn a/empty laye s fomed between wate and soot. The lght ay tavels fom dense medum (wate) to ae medum(a/empty laye). If the angle of ncdence s geate than the ctcal angle, then total ntenal eflecton takes place. Hence metal ball appeas n shnng. 11. Take a glass vessel and pou some glycene nto t and then pou wate up to the bm. Take a quatz glass od. Keep t n the vessel. Obseve the glass od fom the sdes of the glass vessel. (a) What changes do you notce? (b) What could be the easons fo these changes? Take a glass vessel and pou some glycene nto t. Then pou wate up to the bm. Let a quatz glass od be Kept n the vessel. The pat of glass od n the glycene dsappeas and nvsble. As the efactve ndex of glass od and glycene ae same, we can not dentfy the nteface of these two medums. Hee no efacton takes place. But the efactve ndexes of wate and glass od ae dffeent. So the glass od appeas wth bg sze due to efacton. 12. Do actvty-7 agan. How can you fnd ctcal angle of wate? Explan you steps befly. Take a cylndcal tanspaent vessel of 1 lt. Place a con at the bottom of the vessel. Now pou wate untl we get the mage of the con on the wate suface (look at the suface of wate fom a sde). Ths s the phenomenon of total ntenal eflecton. When lght ay passes fom dense medum to ae medum and If the angle of ncdence s moe than ctcal angle, then total ntenal eflecton occus. Ctcal angle of wate : Applyng Snell s law n 1. Sn = n 2. Sn n wate. Sn C = n a. Sn 90 o 1.33 x Sn C = x 1 Sn C =.. = Sn C = Sn 48 o 46 I C = 48 o 46 I = o (appox)

4 16. Explan the efacton of lght though a glass slab wth a neat ay dagam. When lght tavels fom one medum to anothe medum, ts decton changes at the nteface. Ths phenomenon s called efacton. If lght tavels fom ae medum to dense medum, t bends towads the nomal and f lght tavels fom dense medum to ae medum, t bends away to the nomal. The efactng sufaces of glass slab ae paallel to each othe. When lght ay ncdent on one suface of the glass slab, t efacted twce and fnally emeges fom the second suface. At fst efacton t tavels fom ae medum to dense medum. And at the second efacton t tavels fom dense medum to ae medum. ncdent ay shft nomal suface-1 suface-2 s e emegence ay The pependcula dstance between the ncdent ay and emegence ay s called as lateal shft, f the slab s placed hozontally on the plane. The pependcula dstance between the ncdent ay and emegence ay s called as vetcal shft, f the slab s placed vetcally on the plane. The angle between the actual path of ay and efacted ay s called angle of devaton (s). 17. Place an object on the table. Look at the object though the tanspaent glass slab. You wll obseve that t wll appea close to you. Daw a ay dagam to show the passage of lght ay n ths stuaton. actual path of ay Place of the Object s O Place of the Image s O I Aow maks shows the path of the lght ay. 18. What s the eason behnd the shnng of damonds and how do you appecate t? Damonds have hgh efactve ndex (2.42). The ctcal angle of damond s vey less (24.4 o ). By cuttng the faces of damond n such a way that most of the ncdent ays at evey face get total ntenal eflecton. Ths s the eason fo shnng of damonds. 19. How do you appecate the ole of Femat pncple n dawng ay dagams. Femat pncple: Lght selects the path whch takes the least tme to tavel. When the lght gets eflected fom a suface, t selects the paths whch takes the least tme. Ths pncple s vey useful to pepae ay dagams fo the fomaton of mages. 20. A lght ay s ncdent on a-lqud nteface at 45 0 and s efacted at What s the efactve ndex of the lqud? Fo what angle of ncdence wll the angle between eflected ay and efacted ay be 90 0? Case() : angle of ncdence () = 45 o angle of efacton () = 30 o Refactve ndex (n) = = = = = 2 = 1.414

5 Case() : Angle between eflected ay and efacted ay = 90 o 90 o As pe the fgue : + 90 o + = 180 o = 90 o n = = = = tan () tan = n tan = tan = tan 54.7 o = 54.7 o 21. Explan why a test tube mmesed at a cetan angle n a tumble of wate appeas to have a mo suface fo a cetan vewng poston? A test tube mmesed at a cetan angle n a tumble of wate. The suface of the test tube sepaates wate and a. That means the lght ay tavels fom dense medum (wate) to ae medum (a). Ths leads to total ntenal eflecton at some patcula angles of ncdence. So the test tube appeas as a shny mo suface. 22. In what cases does a lght ay not devate at the nteface of two meda? In the followng cases the lght ay does not devate at the nteface. Case(): If the efactve ndexes of two medums ae equal. Case(): When the ncdent ay concdes wth the nomal dawn to the nteface. 23. When we st at a camp fe, objects beyond the fe ae seen swayng. Gve the eason fo t. At camp fe, heat s tansfomed to the suoundngs by convecton. Due to ths pocess, the densty of suoundng a changes contnuously. The efactve ndex contnuously changes slghtly. As a esult the objects beyond the fe ae seen swayng. 24. Why do stas appea twnklng? The lght ays fom the stas tavel though many layes of eath s atmosphee whch ae havng dffeent efactve ndex values. The ays bent many tmes and n andom dectons. As a esult, the stas appea twnklng. 25. Why does a damond shne moe than a glass pece cut to the same shape? Refactve ndex of damond s vey hgh (2.42). It s moe than nomal glass. Due to hgh efactve ndex, ctcal angle fo damond s vey less. So most of the ay ncdent on the damond suface, gets total ntenal eflecton. So t shnes moe. 26. What s the angle of devaton poduced by a glass slab? Explan wth ay dagam. If lght tavels fom ae medum to dense medum, t bends towads the nomal ( > )and f lght tavels fom dense medum to ae medum, t bends away to the nomal ( < ). ncdent ay shft nomal suface-1 suface-2 s actual path of ay e emegence ay The angle between the actual path of ay and efacted ay s called angle of devaton (s). s = A ay of lght tavels fom an optcally dense to ae medum. The ctcal angle of the two meda s c. What s the maxmum possble devaton of the ay? A ay of lght tavels fom an optcally dense to ae medum. The ctcal angle of the two meda s c. The angle of devaton = π 2c