qepansd cate constan veesinr isminimum in medinm ¢, hence the veloc~ sine? ia ei € wi be: ania ig, Why do the bases of houses and trees aps sabe shaky’ during stumme ee Dring sumer the a near the eauth becomes ae therefive, the air near cartl’s surface behaves ta er mea ad the layers a gore eight e- ke pe denser mem, Hence, efferent layers have be active indices. So, the beam of light come feet reactive it aide the asc of the houses and trees gets refracted to the observer, Hence, im ierentanles ana reach the bases appears 10 be shaky. a, Aray of Fight enters front yacuut into a medium of refractive index 4. Ifthe angle of incl. Mence is twice the angle of refraction, then find the angle of incidence “Ans. By Snell’s law Given or tS) objective Type Questions [a] Maltiple-choice Questions : 1, The refractive index of glass with respect to air is 3 then refractive index of air with respect to glass will be: 3 of OF ©2 3 2. An object placed in denser medium is seen from a rarer medium. If h and A be the real and apparent depths of the object, then : (hah (WASH (@k>H (A) None of these. 3. Twinkling of stars is duc to : (@) Light reffacts from denser to the rarer medium () Light reflects through different layers of varying density (©) Stars are in motion (@ Earth is in motion. 4. Ifi, be the critical angle of a liquid, then its retrge- tive index will be : y (@sinj, —(b) cosi, (e) sei, (A) casei. & Which of the following statements is true for total internal reflection : (@) Thelight should move from denser torarer medium () Angle of incidence should be greater than the critical angle. 6 1 9. (©) (a) and (b) both are correct (@) The light should move from rarer to denser me~ dium. ‘The refractive index of glass is minimum for : (a) Violet colour (b) Red colour (©) Yellow colour (d) Blue colour, When light moves from rarer to denser medium. Which of the following does not chang (a) Velocity (b) Amplitude (©) Frequency (d) Wavelength, ‘The brilliance of diamond is due to : (a) Dispersion of light (b) Scattering of light (©) Total internal reflection of light (@) Interference of light. ‘What would happen, if the earth did not have at- mosphere : (a) The duration of the day would be increased (b) The duration of the day would be decreased (©) The duration of the day will remain same (@) The duration of the day has no relation with the atmosphere. Which of the following is not an example of total internal reflection : (@) Twinkling of stars (b) Brilliance of diamond (©) Mirage (@) Shining of air bubble in water,‘716 | Navbodh Physics : Class XII {B] Fill in the Blanks : 1, The speed of light in air is 3x 10% mvs. Ifthe refrac- tive index of glass is 1-5, then the speed of light in B1RSS 18 cen . 2. Critical angle is that value of angle of incidence, for which the angle of refraction is i aya . colour of visible light has maximum velocity in glass. 4, When lighttravels from one medium to another its, : .. femains unchanged, 5. When light moves from rarer to denser medium, then the angle of refraction is . than the angle of incidence. 6. The velocity of light in denser medium is. than that in rarer medium, 7. The different velocities of light in various media is the cause of. .. of light. 8. Due to refraction the stars appear at........ height than their actual attitude, “ves is the prineiple of working of optical fibre. 10. The refiactve index of water is then the refac- tive index of air with respect to water is IC] State True / False : 1. Tfaray of light suffers refraction through several me- dia then on reversing its path it retraces the path in opposite direction, When a ray of light enters from one medium to an- other, its frequency is changed. 3. The different velocities of light in different media is the cause of its refraction. 4. The speed of light for all colours is same, in vacuum, ‘When an object situated in denser medium is viewed from a rarer medium, it appears at more depth, 6. For total internal reflection the angle of incidence should be greater than the critical angle 7. When aay of light falls normally on the interface of the two media, it remains undeviated. 8. When a ray of light travels from rarer to denser me- dium, it moves away from the normal, 9. If the atmosphere does not exist, the duration of the day would be reduced, 10. Total reflecting prism is right-angled isosceles prism, 11. Optical fibres are used in surgery of retina. {D] Match the Columns : L “a om Material Refractive index 1. Glass (flint) (@) 2-42 2. Water (b) 1-0003 3. Air (c) 133 4, Diamond @ RS 5. Glass (crown) © ou 1 Column ‘A’ Column *B Real de; i (a) “pth 1. Refraction ‘Apparent doh 2. Total internal () Total intemal reflection reflection 3. Refractive index (©) Telecommun. cation 4, Optical fibre @) Brilliance o¢ diamond 5. Mirage (©) Different velocitng of light in ditteren media, [E] Write the Auswer in One Word/ One Sentence; 1. What characteristics of light change during reffaciy of light ? 2. When does the Snell’s law fail ? 3. On what factors does the refractive index of a me. dium depend ? 4. Whenmonochromatic light travels from one mediu to another, its wavelength changes but frequency re mains the same, Explain. (@ethi 2011) 5. Ifa ray of light propagates from a rarer to a denser medium, how does its frequency change ? (AU India 2011 ¢) 6. Write the principle on which optical fibre works, (Foreign 2009) 7. Why does diamond sparkle 2 8. Which physical quantity remains same in reaction of light ? 9. Write unit of refractive index. 10. Name the colour for which refractive index is mini mum. 11. Name the colour for whi mum, 12. Does eritical an, 13. Write a relation angle, ich refractive index is maxi- ile depend on colour of light ? between refractive index and critical 14. What will be the value of angle of refraction ifangle ofincidence is equal to the critical angle? ty Short Answer Type Questions |. What is refraction of light 7 State laws of refraction, Write Principle of reversibility. “Sunrise and sunset: takes place on the surface of moon simultaneously.” ‘Why? at is critical angle ? What do you understan Write down the Ver nays nd by total internal reflection ? ‘onditions of total intemal reflection. aapparent depth ofa neee! ee se naaaired by a microscope to be 9:4 em. What is the refractive index of water. If water is replaced by a liquid of refractive index 1-63 up to the same height, by what distance would the microscope have to be more to focus on the needle again ? ieee. ef es ON ep re th surface of water through which light from the but can emerge out ? Refractive index of water s £ Cn sider the bulb to be a point source. ANSWERS Objective Type Questions [A] Multiple-choice Questions 1. (6), 2. (0), 3. (b), 4. (d), 5: (©), 6. (b), 7. (©), 8. (©), 9..(b), 10. (a). [B] Fill in the Blanks 1.2 x10® nvs., 2. 90°, 3. Red, 4. Frequency, 5. Smaller, 6. Less, 7. Refraction, 8. More, 9. Due to total internal refraction of light, 10. 3 . IC] State True / False 1. True, 2. False, 3. Truc, 4. True, 5. False, 6. False, . True, 10, True, 11. True. {D] Match the Columns 1.1. (d), 2. (©), 3. (b), 4. (a), 5. (€). IL. 1. (€), 2. (d), 3. (a), 4. (©), 5+ (0). [E] Answer in One Word/One Sentence 1. Velocity, wavelength and amplitude, 2. When ray of light incident normally on the interface, 3. Nature of pair of media, density, temperature and colour of light Competitive FILE ie., wavelength, 4. Frequency is the characterise property of light, 5. Remains unchanged, 6, Tou internal reflection, 7. Total internal reflection, 8. Fres. uency, 9. No unit, 10. For red colour, 11. For viol 1 sini, colour, 12. Yes, 13. =—— , 14. 90°. Numerical Problems 1. 19-47°,2 «10% m/sec. 2. 2:25 x10" m/sec. 3. 310sec. 4, 2 108 metredsec. 5. 310° metre/sec. 6. 2:2 x 10° sec, 7. 2-66 metre 8. 6cm 9. 4 metre 10, 1-33; 1-7em 1. 353° cm 12, 2-12x10% m/sec. 13. 70-13° 14, 53-12° 15. 2:58 metre”, IMporTANT INFORMATIONS %* =When a beam of light travels from denser to rarer medium the maximum value of angle of refraction is 90°. * Snel * Refractive index may never be zero. Absolute refractive index is always greater than one. law fails when a ray of light falls normally on a refracting surface.‘Objective Type Questions [A] Multiple-choice Questions : 1, An object is placed between F and 2F of a convex lens. The image will form : (@) Virtual, erect and bigger than object (b) Real, inverted and bigger than object (©) Real, inverted and smaller than object (@) Real, inverted and equal to the size of object. 2. Aconvex lens of crown glass (j = 1°52) will act as a diverging lens if it is immersed into the : (a), Water (=133) (b) Liquid (uw =152) (©) Carbon dioxide (7 = 1-66) (d)_ None of these. 3. A convex lens made of glass is immersed into a liquid of refractive index equal to the glass. It will behave like (@) Convex lens (b) Concave lens (©) Plane glass slab (4) None of these, 4. Aconver lens of focal length 20 em is kept in con- tact with a concave lens of focai length 30 cm. The Power of this combination will be : @ 1-67D () -1-67D © 833D @ -833D. 5. The image formed by a diverging (concave) lens is: (@) Always real () Always virtual (©) May be real or virtual depends on the position of object (@) None of these. 6. Two lenses of focal length f, and f; are kept in contact with each other. The focal length of combi- nation will be : @ fh ©) Ath Sth Sh Ath 4) Sh © hi OF 7. The radius of curvature of a planoconvex lens is 12 em and refractive index is 1-5, The focal length of lens will be : (@) 12cm () 15 em (©) 24cm (@ 30cm. 8. For which colour the focal length of a lens will be minimum : (@) Violet (©) Yellow (b) Red (@) Blue. “10. Focal length of a plane glass plate is in the Blanks : 1, In the displacement method to determine the foca, length of convex lens, the distance between tv should be greater than - of focal length 2. The focal length ofa convex lens is 20 cm. It po will be .- dioptre, 3. Unit of power of lens is 4. According to sign convention the length of the lens is called its focal length, 5. Thereciprocal of focal length of a lens is called .. oflens. 6. When a lens is immersed into a liquid then its foe length is 7. When a lens is immersed into a liquid of . . refractive index, then it acs like a plane glass plate 8. Lens maker’s formula is 9. Inconcaye lens the linear magnification is . focal 11. Anair bubble inside the water behaves like a lens, [C] State True/False : 1. The light ray passes through optical centre, emerges out from the lens without deviation. 2. When a lens is immersed into a liquid its power is increased, 3. By convex lens, always real image of an object is ob- tained, 4. Ifa convex lens and a concave lens of equal foce length are kept in contact then this combination will act as a transparent glass plate, 5. By the displacement method, the focal length of com- cave lens cannot be obtained. &. ‘The power ofa lens of focal length 1 em is | diopte. 7. The formula of linear magnification of a lens is m= u 8. Newton's formula xix) = if; is true only for rea images. 9. Inthe combination of lenses, the net magnification is equal to the product of magnification produced by individual lenses, 10. When a convex lens of less focal length is kept in contact with a concave lens of greater focal length, then the combination will act as convex lens.- 7 {o] Match the Columns: 1 Columns ‘A 1, Refraction formula for (a) spherical surface 2, Lens maker's formula 3, Lens equation 4, Power 5. Displacement method 6. Combination of lens 1 ‘A Position of object for convex lens a Position of image 1. Atinfinity (@) Between F and 2F 2. Betwoen 2F and infinity (b) At infinity 3 ALF (© ALF 4, Baween Fand2F (4) Between 2F-and infinity 5. ALF (©) At2F. [E] Write the Answer in One Word/One Sentence : 1. Alens is dipped in water. Willits focal length change ? 2. How does the focal length of convex lens changes when red light incident on it is replaced by violet light 2 (Foreign 2012) 3. For which colour focal length of lens in maximum ? 4. On factor the focal length of a lens depends ? Name them, 5. An object is kept at focus of a concave lens. What is the position of image formed 7 6. When does a lens immersed in a liquid behave like plane glass plate 7 . How does an air bubble inside water behave ? ‘What is the unit of power of a lens ? ‘What is the focal length and power of a glass plate ? A lens disappears when immersed in a transparent + __ liquid. Under what condition it is possible ? 11, When the two focal length of lens are equal ? 12. Can alens be used in a medium of which it is made ? Very Short Answer Type Questions 1. Aconcave lens of refractive index 1:5 is immersed in ‘medium of refractive index 1°65. What is the nature oflens 7 (All India 2015) 6 7. Under what condition we cannot get an image on the 8, 9, Refraction Through Spherteal Surface | 757 ‘A biconvex lens made of a transparent material of refractive index. 1-25 is immersed in water of refractive index 1-33. Will the lens behave as a converging or @ diverging lens ? (All India 2014) A biconvex lens made of a transparent material of refractive index 1-5 is immersed in water of refractive index 1-33 wall the lens behave as converging or 2 diverging lens ? Give reason, (All India 2014) Under what conditions, does a biconvex lens of glass having, a certain refractive index act as a plane glass sheet when immersed in aliquid. (Delhi 2012) ‘A plass lens of refractive index 1-45 disappears when immersed in a liquid. What is the value of refractive index of liquid ? (Delhi 2010) Define, power of lens and write its SI unit. screen due to convex lens ? When can we use convex lens as 2 magnifier ? ‘The letters of a book appear to be magnified if seen through a fused bulb containing water. Why ? Short Answer Type Questions Violet rays are incident on a thin lens. Ifitis replaced by red colour, then by giving reason justify how will the power of lens change ? (Foreign 2004) How will the focal length and nature oflens change if the refractive index of the liquid in which it is dipped is: (less than refractive index of lens ? i) more than refractive index of lens ? (iii) equal to refractive index of lens ? Define the following terms in relation to lens : (i)Principal axis, (ii) Optical centre, (iii) First principal focus, (iv) Second principal focus. Explain withthe help of ray diagram how does convex lens acts as converging lens ? Explain with ray diagram how a concave lens acts as diverging lens ? ‘An object is placed in front of a convex lens. By drawing a ray diagram, show the formation of image, its position and nature if the object is placed : (i between optical centre and focus of lens. (ii) at focus of lens. ‘An object is placed in front of a concave lens. Draw the ray diagram to show the formation of image. What will happen to the size of image if the object is moved away from it? Ifconvex lens of focal length fis kept in contact with ‘concave lens of focal length f;. Find the focal length of the combination. (All India 2013) Or : ‘Two lenses of focal lengths f, and f; are in contact, with each other, if resultant focal length of this combination is ‘F” then prove that : Lyd, A Poke A!wr a on wt fe OUDIC,, large and real image could formed ? 15. Where should be the object placed in front of convex Jens of focal length 15 cm so that a double, large and virtual image of object could formed ? 16, Where should be the object placed in front of aconvex Jens of focal length 9 cm, so that 1-5 times larger im- age could formed ? Objective Type Questions [A] Multiple-choice Questions 1. (c), 2. (c), 3. (c), 4. (a), 5. (b), 6. (d), 7. (0), 8. (2). [B} Fill in the Blanks 1. four times, 2. +5, 3. dioptre, 4. second, 5. power, 1 jee a w-0(2-2}, 7 RR 9. less than one, 10. infinite, 11. concave.” [C] State True/False 1. True, 2. False, 3. False, 4. True, 5. True, 6. False, 7. False, 8. True, 9. True, 10. True. (D] Match the Columns L1.(@), 2. (0), 3. (b), 4. (9, 5. @), 6 @). IL 1. (0), 2. (a), 3. (€), 4. (d), 5+ (b). 6. increased, 7. equal, 8. PWT OT) 4. Power of a lens is +10 D, when it is immersed in a liquid, then it works as concave lens of focal length 50 cm. If refractive index of material of lens is 15, then calculate refractive index of liquid ? A concave lens has been kept in contact with convex lens of focal length 20 cm. This combination works as a convex lens of focal length 50 cm. Find power of concave lens, 25. [E] Answer in One Word/One Sentence 1. Focal length will increase, 2. Focal length will decrease, 3. For red colour it is maximum, 4. Material of lens, Colour of light, Radius of curvature of curved surfaces and refractive indices of media on both sides of alens, 5. In between focus and optical centre, 6. Re- fractive index of liquid should be equal to that of material of lens, 7. Concave lens, 8. Dioptre, 9. Focal length is infinity and power will be zero, 10. When refra-ctive indices of material of lens and liquid are same, 11. When medium on both sides is identical, 12. No. Very Short Answer Type Questions 1, It will act as a convex lens. alt; >q Hg, So the sign off, will change, so concave lens will behave as a convex lens.