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.

1. DISPERSION IN LIGHTDISPERSION IN LIGHT
A powerpoint presentation by
Brajesh Kumar Chaudhary
Faculty of Physics
Resonance Eduventures Pvt Ltd.

2. WHAT IS Dispersion ?
Dispersion is the separation of white light into
a spectrumm by the process of refraction.
The rainbow - An example of dispersion caused by water

3. DISPERSION WITH A PRISMDISPERSION WITH A PRISM
When a white light is passed
through a prism , white light
is separeted into different
components(colors) and they
spread into different
directions.
•Violet is deviated the
most
•Red is deviated the least
R
O
Y
G
B
I
V

4. REA SON FOR DISPERSIONREA SON FOR DISPERSION
Waves of different wavelength move with same speed in
vacuum, 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 ?
violet
red
wavelength
700 nm
400 nm

5. •When light propagates through a material the speed of light is reduced owing
to the response of the medium to the electric and magnetic fields.
•Although the wave speed changes the energy of the wave does not change. As
E = hf , the frequency remains constant.
•This means that the wavelength must reduce. The effective wavelength in the
material is given by λ/n.this means r.i. of medium depends slightly on
wavelength also.
REASON FOR DISPERSIONREASON FOR DISPERSION

6. CAUCHY’S FORMULACAUCHY’S FORMULA
2
B
n A
λ= +
Where, n= r.i. of a transparent medium
λ = wavelength of light in vacuum
,R V V Rn nλ λ〉 ∴ 〉Q
THUS, violet light deviates more than red light, this is the reason of
dispersion

7. QUANTIFYING DISPERSION-ANGLE OF DISPERSIONQUANTIFYING DISPERSION-ANGLE OF DISPERSION
The angle between the rays of the extreme colors(violet &
red) in the refracted (dispersed)light is called angle of
dispersion(Ө).
V Rθ δ δ= −
Where,Ө=angle of dispersion
=deviation in violet color
=deviation in red color
Vδ
Rδ
white
λ = 656.3 nmλ = 486.1 nm
Ө

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. CONCEPT QUESTION#1CONCEPT QUESTION#1
glass
W
hite ray
White light is incident on one face of a parallel glass slab, how much
is dispersion after ray is passed from second parallel surface?
air air

10. Answer to Concept Q#1Answer to Concept Q#1
All are
Parallel
to each
other
So, dispersion is zero in this case.

11. CONCEPT QUESTION # 2CONCEPT QUESTION # 2
WHITE RAY
White ray is incident on a hollow prism as shown, how much is
dispersion if
(a)Medium inside & outside the prism is same.
(b)Medium inside & outside the prism are not same.

12. DISPERSIVE POWER OF MEDIUMDISPERSIVE POWER OF MEDIUM
1 _ _ _
r
V R V R
n
y Y
n n ang dispersion
n dev of mean ray
δ δ
ω
δ
− −
= = =
−
Note – ω is property of medium
if is not given in the problem, thenyn
2
V R
Y
n n
n
+
=

13. DISPERSION WITHOUT DEVIATIONDISPERSION WITHOUT DEVIATION
CONDITION
i.e.
Sign convention-
In total deviation add deviations of similarly oriented prism &
subtract deviation of oppositely oriented prism
0totalδ =
' '
( 1) ( 1) ..... 0y yn A n A− ± − + =

14. DISPERSION WITHOUT DEVIATIONDISPERSION WITHOUT DEVIATION

15. DEVIATION WITHOUT DISPERSIONDEVIATION WITHOUT DISPERSION
CONDITION-
i.e.
Sign convention- Angle of dispersion for similarly oriented prism
is positive & for oppositely oriented prism is negative.
0totalθ =
' ' '
( ) ( ) ........ 0V R V Rn n A n n A− ± − ± =

16. A2
Crown
A1
blue
red
n1
n2
Ө
white
Seen
As
White
light
AN ACHROMATIC DOUBLET-DEVIATION WITHOUTAN ACHROMATIC DOUBLET-DEVIATION WITHOUT
DISPERSIONDISPERSION

17. PROBLEM#1PROBLEM#1
The refractive indices of flint glass for red & violet
light are 1.613 & 1.632 respectively. Find the angular
dispersion produced by a thin film of flint glass
having refracting angle .0
5

18. PROBLEM#2PROBLEM#2
Refractive 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. If two prism are combined as shown in the fig, find the
total angular dispersion & angle of deviation suffered by
a white ray of light incident on combination.
0
2
0
4
White ray
' '
1.7, 1.5V Rn n= =1.5, 1.4V Rn n= =
PROBLEM #3PROBLEM #3

20. Two thin prism are combined to form an achromatic doublet. For
1st
prism,
& for second prism, . Find prism
angle of second prism.
0
1.42, 1.35, 1.4, 4V R Yn n n A= = = =
' ' '
1.9, 1.7, 1.8v R Yn n n= = =
PROBLEM#4PROBLEM#4

21. A crown glass prism of angle is to be combined with a flint
prism in such a way that the mean ray passes through un
deviated.
Find
(a)The apex angle of the flint glass needed
(b)The angular dispersion produced by the given combination
when white light is incident on it.
Data- r.i for red , yellow & violet light are 1.5, 1.6 & 1.6
respectively for crown glass & 1.8, 2.0 , & 2.2 for flint glass.
0
5
PROBLEM#5PROBLEM#5