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Dispersion of light




This PPT gives an elementary idea about dispersion. The dispersion through prism is discussed in some details & combination of prisms are made to make either dispersion or deviation to be equal to zero.

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  1. 1. DISPERSION IN LIGHTDISPERSION IN LIGHTA powerpoint presentation byBrajesh Kumar ChaudharyFaculty of PhysicsResonance Eduventures Pvt Ltd.
  2. 2. WHAT IS Dispersion ?Dispersion is the separation of white light intoa spectrumm by the process of refraction.The rainbow - An example of dispersion caused by water
  3. 3. DISPERSION WITH A PRISMDISPERSION WITH A PRISMWhen a white light is passedthrough a prism , white lightis separeted into differentcomponents(colors) and theyspread into differentdirections.•Violet is deviated themost•Red is deviated the leastROYGBIV
  4. 4. REA SON FOR DISPERSIONREA SON FOR DISPERSIONWaves of different wavelength move with same speed invacuum, but with different speeds in a medium.What is the real difference between red and violet light ?What is the real difference between red and violet light ?violetredwavelength700 nm400 nm
  5. 5. •When light propagates through a material the speed of light is reduced owingto the response of the medium to the electric and magnetic fields.•Although the wave speed changes the energy of the wave does not change. AsE = hf , the frequency remains constant.•This means that the wavelength must reduce. The effective wavelength in thematerial is given by λ/n.this means r.i. of medium depends slightly onwavelength also.REASON FOR DISPERSIONREASON FOR DISPERSION
  6. 6. CAUCHY’S FORMULACAUCHY’S FORMULA2Bn Aλ= +Where, n= r.i. of a transparent mediumλ = wavelength of light in vacuum,R V V Rn nλ λ〉 ∴ 〉QTHUS, violet light deviates more than red light, this is the reason ofdispersion
  7. 7. QUANTIFYING DISPERSION-ANGLE OF DISPERSIONQUANTIFYING DISPERSION-ANGLE OF DISPERSIONThe angle between the rays of the extreme colors(violet &red) in the refracted (dispersed)light is called angle ofdispersion(Ө).V Rθ δ δ= −Where,Ө=angle of dispersion=deviation in violet color=deviation in red colorVδRδwhiteλ = 656.3 nmλ = 486.1 nmӨ
  8. 8. For prism of small angle(A) & with small angle of incidence(i)( 1)n Aδ = −So, angle of dispersion(Ө) becomes( )V R V Rn n Aθ δ δ= − = −Deviation of beam(also called mean deviation)( 1)Y Yn Aδ δ= = −
  9. 9. CONCEPT QUESTION#1CONCEPT QUESTION#1glassWhite rayWhite light is incident on one face of a parallel glass slab, how muchis dispersion after ray is passed from second parallel surface?air air
  10. 10. Answer to Concept Q#1Answer to Concept Q#1All areParallelto eachotherSo, dispersion is zero in this case.
  11. 11. CONCEPT QUESTION # 2CONCEPT QUESTION # 2WHITE RAYWhite ray is incident on a hollow prism as shown, how much isdispersion if(a)Medium inside & outside the prism is same.(b)Medium inside & outside the prism are not same.
  12. 12. DISPERSIVE POWER OF MEDIUMDISPERSIVE POWER OF MEDIUM1 _ _ _rV R V Rny Yn n ang dispersionn dev of mean rayδ δωδ− −= = =−Note – ω is property of mediumif is not given in the problem, thenyn2V RYn nn+=
  13. 13. DISPERSION WITHOUT DEVIATIONDISPERSION WITHOUT DEVIATIONCONDITIONi.e.Sign convention-In total deviation add deviations of similarly oriented prism &subtract deviation of oppositely oriented prism0totalδ = ( 1) ( 1) ..... 0y yn A n A− ± − + =
  15. 15. DEVIATION WITHOUT DISPERSIONDEVIATION WITHOUT DISPERSIONCONDITION-i.e.Sign convention- Angle of dispersion for similarly oriented prismis positive & for oppositely oriented prism is negative.0totalθ = ( ) ( ) ........ 0V R V Rn n A n n A− ± − ± =
  17. 17. PROBLEM#1PROBLEM#1The refractive indices of flint glass for red & violetlight are 1.613 & 1.632 respectively. Find the angulardispersion produced by a thin film of flint glasshaving refracting angle .05
  18. 18. PROBLEM#2PROBLEM#2Refractive index of glass for red & violet colors are 1.50& 1.60 respectively . Find(a)The refractive index for yellow color(b)Dispersive power of the medium.
  19. 19. If two prism are combined as shown in the fig, find thetotal angular dispersion & angle of deviation suffered bya white ray of light incident on combination.0204White ray 1.7, 1.5V Rn n= =1.5, 1.4V Rn n= =PROBLEM #3PROBLEM #3
  20. 20. Two thin prism are combined to form an achromatic doublet. For1stprism,& for second prism, . Find prismangle of second prism.01.42, 1.35, 1.4, 4V R Yn n n A= = = = 1.9, 1.7, 1.8v R Yn n n= = =PROBLEM#4PROBLEM#4
  21. 21. A crown glass prism of angle is to be combined with a flintprism in such a way that the mean ray passes through undeviated.Find(a)The apex angle of the flint glass needed(b)The angular dispersion produced by the given combinationwhen white light is incident on it.Data- r.i for red , yellow & violet light are 1.5, 1.6 & 1.6respectively for crown glass & 1.8, 2.0 , & 2.2 for flint glass.05PROBLEM#5PROBLEM#5
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This PPT gives an elementary idea about dispersion. The dispersion through prism is discussed in some details & combination of prisms are made to make either dispersion or deviation to be equal to zero.


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