Wave Optics - 1 PPT Physics Class 12

 Page 1


WAVE  OPTICS - I
1. Electromagnetic Wave
2. Wavefront
3. Huygens’ Principle
4. Reflection of Light based on Huygens’ Principle
5. Refraction of Light based on Huygens’ Principle
6. Behaviour of Wavefront in a Mirror, Lens and Prism
7. Coherent Sources
8. Interference
9. Young’s Double Slit Experiment
10.Colours in Thin Films
Page 2


WAVE  OPTICS - I
1. Electromagnetic Wave
2. Wavefront
3. Huygens’ Principle
4. Reflection of Light based on Huygens’ Principle
5. Refraction of Light based on Huygens’ Principle
6. Behaviour of Wavefront in a Mirror, Lens and Prism
7. Coherent Sources
8. Interference
9. Young’s Double Slit Experiment
10.Colours in Thin Films
0
Electromagnetic Wave:
X
E
0
B
0
Y
Z
Wave is propagating along X – axis with speed c = 1 / vµ
0
e
0
For discussion of optical property of EM wave, more significance is given to 
Electric Field, E.  Therefore, Electric Field is called ‘light vector’.
1. Variations in both electric and magnetic fields occur simultaneously.  
Therefore, they attain their maxima and minima at the same place and at 
the same time.
2. The direction of electric and magnetic fields are mutually perpendicular 
to each other and as well as to the direction of propagation of wave.
3. The speed of electromagnetic wave depends entirely on the electric and 
magnetic properties of the medium, in which the wave travels and not on 
the amplitudes of their variations.
Page 3


WAVE  OPTICS - I
1. Electromagnetic Wave
2. Wavefront
3. Huygens’ Principle
4. Reflection of Light based on Huygens’ Principle
5. Refraction of Light based on Huygens’ Principle
6. Behaviour of Wavefront in a Mirror, Lens and Prism
7. Coherent Sources
8. Interference
9. Young’s Double Slit Experiment
10.Colours in Thin Films
0
Electromagnetic Wave:
X
E
0
B
0
Y
Z
Wave is propagating along X – axis with speed c = 1 / vµ
0
e
0
For discussion of optical property of EM wave, more significance is given to 
Electric Field, E.  Therefore, Electric Field is called ‘light vector’.
1. Variations in both electric and magnetic fields occur simultaneously.  
Therefore, they attain their maxima and minima at the same place and at 
the same time.
2. The direction of electric and magnetic fields are mutually perpendicular 
to each other and as well as to the direction of propagation of wave.
3. The speed of electromagnetic wave depends entirely on the electric and 
magnetic properties of the medium, in which the wave travels and not on 
the amplitudes of their variations.
Wavefront:
A wavelet is the point of disturbance due to propagation of light.
A wavefront is the locus of points (wavelets) having the same phase of 
oscillations.
A line perpendicular to a wavefront is called a ‘ray’.
Spherical 
Wavefront 
from a point 
source
Cylindrical 
Wavefront 
from a linear 
source
Plane 
Wavefront
Pink Dots – Wavelets
Blue Envelope– Wavefront      
Red Line – Ray 
•
Page 4


WAVE  OPTICS - I
1. Electromagnetic Wave
2. Wavefront
3. Huygens’ Principle
4. Reflection of Light based on Huygens’ Principle
5. Refraction of Light based on Huygens’ Principle
6. Behaviour of Wavefront in a Mirror, Lens and Prism
7. Coherent Sources
8. Interference
9. Young’s Double Slit Experiment
10.Colours in Thin Films
0
Electromagnetic Wave:
X
E
0
B
0
Y
Z
Wave is propagating along X – axis with speed c = 1 / vµ
0
e
0
For discussion of optical property of EM wave, more significance is given to 
Electric Field, E.  Therefore, Electric Field is called ‘light vector’.
1. Variations in both electric and magnetic fields occur simultaneously.  
Therefore, they attain their maxima and minima at the same place and at 
the same time.
2. The direction of electric and magnetic fields are mutually perpendicular 
to each other and as well as to the direction of propagation of wave.
3. The speed of electromagnetic wave depends entirely on the electric and 
magnetic properties of the medium, in which the wave travels and not on 
the amplitudes of their variations.
Wavefront:
A wavelet is the point of disturbance due to propagation of light.
A wavefront is the locus of points (wavelets) having the same phase of 
oscillations.
A line perpendicular to a wavefront is called a ‘ray’.
Spherical 
Wavefront 
from a point 
source
Cylindrical 
Wavefront 
from a linear 
source
Plane 
Wavefront
Pink Dots – Wavelets
Blue Envelope– Wavefront      
Red Line – Ray 
•
Huygens’ Construction or Huygens’ Principle of Secondary 
Wavelets:
S
New Wavefront 
(Spherical)
New 
Wave-
front 
(Plane)
.
.
.
.
.
.
.
.
(Wavelets - Red dots on the wavefront) 
1. Each point on a wavefront acts as a fresh source of disturbance of light.
2. The new wavefront at any time later is obtained by taking the forward 
envelope of all the secondary wavelets at that time.
Note: Backward wavefront is rejected.  Why?
.
.
.
.
.
.
.
.
.
Amplitude of secondary wavelet is proportional to ½ (1+cos?).  Obviously, 
for the backward wavelet ? = 180° and (1+cos?) is 0. 
•
Page 5


WAVE  OPTICS - I
1. Electromagnetic Wave
2. Wavefront
3. Huygens’ Principle
4. Reflection of Light based on Huygens’ Principle
5. Refraction of Light based on Huygens’ Principle
6. Behaviour of Wavefront in a Mirror, Lens and Prism
7. Coherent Sources
8. Interference
9. Young’s Double Slit Experiment
10.Colours in Thin Films
0
Electromagnetic Wave:
X
E
0
B
0
Y
Z
Wave is propagating along X – axis with speed c = 1 / vµ
0
e
0
For discussion of optical property of EM wave, more significance is given to 
Electric Field, E.  Therefore, Electric Field is called ‘light vector’.
1. Variations in both electric and magnetic fields occur simultaneously.  
Therefore, they attain their maxima and minima at the same place and at 
the same time.
2. The direction of electric and magnetic fields are mutually perpendicular 
to each other and as well as to the direction of propagation of wave.
3. The speed of electromagnetic wave depends entirely on the electric and 
magnetic properties of the medium, in which the wave travels and not on 
the amplitudes of their variations.
Wavefront:
A wavelet is the point of disturbance due to propagation of light.
A wavefront is the locus of points (wavelets) having the same phase of 
oscillations.
A line perpendicular to a wavefront is called a ‘ray’.
Spherical 
Wavefront 
from a point 
source
Cylindrical 
Wavefront 
from a linear 
source
Plane 
Wavefront
Pink Dots – Wavelets
Blue Envelope– Wavefront      
Red Line – Ray 
•
Huygens’ Construction or Huygens’ Principle of Secondary 
Wavelets:
S
New Wavefront 
(Spherical)
New 
Wave-
front 
(Plane)
.
.
.
.
.
.
.
.
(Wavelets - Red dots on the wavefront) 
1. Each point on a wavefront acts as a fresh source of disturbance of light.
2. The new wavefront at any time later is obtained by taking the forward 
envelope of all the secondary wavelets at that time.
Note: Backward wavefront is rejected.  Why?
.
.
.
.
.
.
.
.
.
Amplitude of secondary wavelet is proportional to ½ (1+cos?).  Obviously, 
for the backward wavelet ? = 180° and (1+cos?) is 0. 
•
Laws of Reflection at a Plane Surface (On Huygens’ Principle):
i
i
r
r
A
B
C
D
N
N
AB – Incident wavefront  
CD – Reflected wavefront    
XY  – Reflecting surface
E
F
G
If c be the speed of light, t 
be the time taken by light to 
go from B to C or A to D or 
E to G through F, then
EF
t  = 
c
+
FG
c
AF sin i
t  = 
c
+
FC sin r 
c
For rays of light from different parts on the incident wavefront, the values of 
AF are different. But light from different points of the incident wavefront 
should take the same time to reach the corresponding points on the 
reflected wavefront.
So, t should not depend upon AF.  This is possible only if sin i – sin r = 0.
i.e. sin i = sin r or i = r
X
Y
AC sin r + AF (sin i – sin r)
t  = 
c