Download Electromagnetic Waves

Electromagnetic Waves
Part II
1
So far
 We have discussed
 The nature of EM waves
 Some of the properties of EM waves.
 Now we will discuss
 EM waves and optics
 Mirrors
 Lenses
 Applications
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3
Reflection & Refraction
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Reflection
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Light Reflection
surface normal
incident ray
same
angle
exit ray
 Angles of incidence measured from the NORMAL to the
mirror.
 For reflection – The angle of incidence equals the angle of
reflection.
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Principle of Least Time
A
too long
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shortest path;
equal angles
B
The light rays
appear to come
from behind the
mirror.
An image is virtual if the light rays from
a point on the object are directed as if
they diverged from a point on the
image, even though the rays do not
actually pass through the image point.
Your eye focuses the
diverging rays reflected
by the mirror.
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PHY102
Mirror, Mirror, on the wall, how big do you
have to be to see it all? The strange world of
images.
“real” you
“image” you
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The speed of light
 Light travel’s at a speed c in a vacuum.
 In an actual material, it travels a bit slower, at a velocity of v.
 The speed of light depends on the material through which it
is traveling.
 DEFINITION – INDEX OF REFRACTION (or refractive
index):
velocityof light in a vacuum
n
1
velocityof light in a transparant medium
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Index of Refraction
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Notice
c
v
v  f
n
n
For two
materials
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c
c 1 vacuum


f f 

vacuum
n1 
1
vacuum
n2 
2
divide
n1 2

n2 1
Huygen’s Principle
Each point on a wavefront acts
as a secondary source of spherical
waves that progress from the
source at the speed of light (whatever
it may be).
A spherical wave with very large radius,
behaves as a plane wave.
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14
.
Not there yet!
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PHY102
The Wave Nature of Light
 The law of refraction is explained by Huygen’s Principle
The little wavelets move slower in
medium 2 than in medium 1. Doing the
tangent shows how the wave fronts bend.
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Some Geometry


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
    900
    900

 
The Wave Nature of Light
sin 1 v1 c / n1 n2



sin  2 v2 c / n2 n1
n1 sin 1  n2 sin  2
2 v2t v2 c / n2 n1




1 v1t v1 c / n1 n2
ADC   2
BAD  1
v1t
v2t
sin 1 
and sin  2 
AD
AD
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If medium 1 is air, then
n1 1 and v1  c and 1  
 n   /n
Snell’s Law of Refraction
n1 sin 1  n2 sin  2
Law of Reflection
1   2
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Another View
Of Reflection
(Huygen’s)
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Both Together
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nb  na
na Sin critical  nb sin(900 )  nb
nb
Sin critical 
na
At the critical angle and
beyond, only reflection is
possible.
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Total Reflection – Optical Fiber
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A cross section of a
submarine communications
cable.
1 - Polyethylene
2 - Mylar tape
3 - Stranded steel wires
4 - Aluminium water barrier
5 - Polycarbonate
6 - Copper or aluminum
tube
7 - Petroleum jelly
8 - Optical fibers
Lengths of 100 KM are possible before amplification is necessary.
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Thousands of conversations can be carried on a single fiber.
Waves on a string.
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dichroism
80%
1% of opposite polarization
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Mirage
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