710 | Navbodh Physics : Class NL Fig. 9(1)-27, Deflecting rays through 180° 3. Inverting an image or Erecting prism: When the rays of light parallel to face AC are made to inci- dent on face AB, Then after refraction they face 4 nile of incideree rays suffer ty, in greater than the {al internal reflection, After refraction theouyh far, BC thene arallel to AC, But, the imapy formed is inverted as shovm in Fig, 9(B)2%, Fig. 90)28, faverting an image ‘These prisms are used in Projection lantern Difference between Plane mirror and Total Reflecting prism Plane Mirror ‘Total Reflecting Prism 1. They require polishing. 2. It absorbs some light falling on it. Even quality mirrors reflect 90-95% of light falling on them. 3. Due to multiple reflection in thick plane mirrors . Polishing is not required. . It does not absorb light falling on it. Near by 100% of light is reflected from it. . Only one image is formed the image is bright blurred image is formed. de., intense, NUMER (TYPE A] Based on Refraction of Light Formulae used :(i) = a, sinr dues, where c= 3%108 m/s, 1 (iv) aly =— aw ii) uaa =, Ne Ha (©) abe Xb Xp Hg =I. Example 1. Speed of light in air is 3x 10% m/s, Calculate the speed in glass with refractive index 15, Sol. Given : c= 3x10" ms; w=1+5 .L EXAMPLES Example 2. Speed of light in glass (refractive index 1:5) is 2x10" m/s. Calculate the speed in uum. Sol. Given : = 1-5 and v= 2x10'm/s As p=S c= yxv=1-5x2x10! v c= 3x 108s. Ans, Example 3. The refractive index of glass is 15. and that of water is 1-3. If the speed of light i water is 2-25 x 10° m/s, whats the speed of light ia glass, Sol. Given : ayy = 1-5, athy =13, vy = 225x108 m/s ¥, But why EE = Me, aty Ye 225x108 = 225x10| is)ase 1S Np POS 1O" n/n Aus. Example 4 Atiht ray rayels drought a 200m thlek glass lab, Cateutate he time taken hy Ht t0 travel normal (0 the glass slab Sol, Given : Thicknoss d= 200m, jr 1+ co S10" m/w c £ As pes op Vem y “u 26108 nits, Distance hut bh Time Speed = my Distance Time * Speed Ans. Example 5. A ray of light of frequency 510" Iz. Ispassing through a liquid. The wavelength of light measured inside the liquid fs found to be 450 nm. Calculate the refractive Index of quid. Sol. Given: v= 5x10!Hz, 4, = 450nm = 450x107 m, so"? ry palided v vA 3x10" fe KIS 5133, Aus 5x10" x450x107 ample 6. A ray of light falls on the rectangu- lar glass slab at an angle of refraction of 30°. Cal- (ulate the angle of incidence. Sol. Given: jo 1°55 r= 30° sing By Snell’ == = sini = sin y Sel’ law = => sind = snr 1 5 sind 1: $xsin30? = 1-545 = 0°75 = f= sin"'(0-75) = 48-5°- Ans, Kefraction Through Mane Surface | [EVPE, HY axed on Real and Apparent Depth i ample 7, A fish appear at a depth of Im, If ei) need He) Ho jie dai Vormutae the refractive Index of water ts of fish 3 + Hind the wetual post So. Given : = tm, gro ue Mv 4 4 he ph = Vee So 1633 m, Ans. Ctr 1-33 ple 8, A fish observes a bird flying at a of 30 em above the surface of water and the Md observes that the fish Is 30 em below the 4 water surface, If o/4w = 3+ then what will be the actual height of bird and actual depth of fish with respect {0 water level ? Sol. First ease : nwt wa Te > hey pty xh = xh! ably 4 Given : aMy "7 and hi= 300m he 330 22-Sem Ans. = 44302 400m, Ans. Example 9. The Velocity of light in glass is 210° m/s and that in air is 3x 10" ns, By how much would an ink dot appear to be raised, when covered by a glass plate 6-0 em thick? Sol. Here y= 2x 108 mvs, c= 3x10°mvs, 3x10 v 2x10 Real depth = 6:0 cm, ye 15712 | Navbodh Physics : Real depth | . Apparent depth & a 4em, u 15 Distance through which ink dot appears to be raised = 6-0—4-0= 2 cm. A Example 10, A tank is filled with water up to a height of 12-4 em. The apparent depth of a needle lying at the bottom of the tank is measured by a microscope to be 9:3 cm. What is the refractive index of water ? If water is replaced by n liquid of refractive index 1-6 up to the same height, by what distance would the microscope have to be moved to focus on the needle again ? (Delhi 2009 C) Sol. First case : Given : Real depth=/= 12-4 cm Apparent depth h’= 9:3 em Ans. 124 6 = 7-75 em, ++ Displacement of microscope =93-775 = 1-55 em. Ans. {TYPE C] Based on Total Internal Reflection 1 Formula use Example 11. The refract index of material is V2. What will be the value of critical angle for it ? Sol. Given: w= V2 Formula : => sini, =sin4s° Ans, Example 12. The velocity of light in air is 3 x 10° m/s and that in a liquid is 1-5 * 10° nv/s. If light travels from liquid to air, then find the critical angle for the liquid. Sol. Given: y, =1+5x 108 m/s vq =3x108 mvs Yq __3x108 Formula: of!" 7-55 108” at Now, abt sink, = sing ==) esin30° ofp 2 i, =30°. Ans, Example 13. A liquid is filled in a container up toa depth of 25 cm. What will be the apparent depth of liquid of its refractive index is 1-25 ? Also find the critical angle for liquid. Sol. Given : = 25 em, v= 1-25 Realdepth _ A ‘Apparentdepth 45 Lage zw 5 Now, using the formula Ans Example 14. A monochromatic light travels with a velocity of 2-225 x 10° m/s ina transparent medium. Find the critical angle. (All India 2009 C) Sol. Given: 22x10" mis c=3x10° ms c__ 3x10* 2:225x108