Subject Unit: Course Material
Subject Unit: Course Material
COURSE MATERIAL
UNIT 4
COURSE B.TECH
SEMESTER 2-2
K.Upendra Raju
PREPARED BY D.Srilatha
(Faculty Name/s) K.R.Surendra
V.Omkar Naidu
Version V-1
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1. Course Objective
The objectives of this course is to
1. To analyse reflection and refraction of electromagnetic waves propagated in
normal and oblique incidences
2. Prerequisites
Students should have knowledge on
1. Electric and Magnetic fields
2. Basic Mathematics
3. Syllabus
UNIT IV
Reflection and Refraction of Plane Waves – Normal and Oblique Incidences, for both
Perfect Conductor and Perfect Dielectrics, Brewster Angle, Critical Angle and Total
Internal Reflection, Surface Impedance, Poynting Vector, and Poynting Theorem –
Applications, Power Loss in a Plane Conductor, Illustrative Problems.
4. Course outcomes
1. Understand principles of reflections and refraction for different incidences (L1)
2. State concepts of power flow using Poynting vector (L2)
3. Calculate Brewster angle, power flow and surface impedance (L3)
PO1 PO2 PO3 PO4 PO5 PO6 PO7 PO8 PO9 P10 PO11 PO12 PSO1 PSO2
CO1 3 3 2 2 3 2 2
CO2 3 3 2 2 3 2 2
CO3 3 3 2 2 3 2 2
C04 3 3 2 2 3 2 2
6. LESSON PLAN
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8. LECTURE NOTES
1.1 INTRODUCTION
When a plane wave from one medium meets a different medium, it is partly
reflected and partly transmitted. The proportion position of the incident wave that is
reflected or transmitted depends on the parameters ε, μ, and σ of the two media
involved. The incident wave can meet the boundary between two media either
normally or with some angle (oblique)
1.2 REFLECTION OF A PLANE WAVE AT NORMAL INCIDENCE:
Suppose that a plane wave propagating along the +Z direction is incedent
normally on the boundary Z = 0 between medium 1(Z < 0) characterized by σ 1, ε1, μ1
and medium 2 (Z > 0) characterized by σ2, ε2, μ2 as shown in the figure (4.1). In the
figure subscripts i, r and t denotes incident, reflected and transmitted waves
respectively.
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General case:
Incident Wave: (⃗⃗⃗ , ⃗⃗⃗⃗ ) is travelling along + ⃗⃗⃗⃗ in medium 1 without considering the
time factor ejωt. We can write
⃗⃗⃗ ( ) ⃗⃗⃗⃗ and …………… (4.1)
⃗⃗⃗⃗ ( ) ⃗⃗⃗⃗ …………… (4.2)
= ⃗⃗⃗⃗
Reflected Wave: (⃗⃗⃗⃗ , ⃗⃗⃗⃗ ) is travelling along - ⃗⃗⃗⃗ in medium 1 without considering the
time factor ejωt. We can write
⃗⃗⃗⃗ ( ) ⃗⃗⃗⃗ and …………… (4.3)
⃗⃗⃗⃗ ( ) ( ⃗⃗⃗⃗ ) …………… (4.4)
= ⃗⃗⃗⃗
Transmitted Wave: (⃗⃗⃗ , ⃗⃗⃗⃗ ) is travelling along + ⃗⃗⃗⃗ in medium 1 without considering
the time factor ejωt. We can write
⃗⃗⃗ ( ) ⃗⃗⃗⃗ and …………… (4.5)
⃗⃗⃗⃗ ( ) ( ⃗⃗⃗⃗ ) …………… (4.6)
= ⃗⃗⃗⃗
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Where Eio, Ero, Eto are respectively the magnitudes of the incident, reflected and
transmitted electric field at Z = 0. The total field in medium 1 consists of both the
incident and reflected fields, whereas medium 2 has only transmitted field. i.e.,
⃗⃗⃗⃗ ⃗⃗⃗ ⃗⃗⃗⃗ ⃗⃗⃗⃗ ⃗⃗⃗⃗ ⃗⃗⃗⃗
⃗⃗⃗⃗ ⃗⃗⃗ ⃗⃗⃗⃗ ⃗⃗⃗⃗
…………… (4.10)
…………… (4.12)
Note that
1. 1 + =
2. Both and are dimensionless and may be complex
3. 0 ≤ l l ≤ 1
The above case is considered as the general case.
Let us consider a special case when medium 1 is a perfect dielectric (lossless
σ1=0) and medium 2 is a perfect conductor (σ2=∞). For this case η2=0; hence Γ = -1
and = 0, showing that the wave is totally reflected. This should be expected
because fields in a perfect conductor must vanish, so there can be no transmitted
wave (E2 = 0). The totally reflected wave combines with the incident wave to form a
standing wave.
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A standing wave does not travel, it consists of two travelling waves (E i and Er) of
equal amplitudes but in opposite directions. Combining equ.(4.1) and (4.3) gives the
standing wave in medium 1 is
E1s = Eis + Ers = + (4.13)
when media 1 and 2 are both lossless we consider another special case (σ1 = 0 =
σ2). In this case η1 and η2 are real so that Γ and . Let us consider the following cases:
CASE A
If η2 > η1, Γ > 0, Again there is a standing wave in medium 1 but there is also a
transmitted wave in medium 2. However, the incident and reflected waves have
amplitudes that are not equal in magnitude. It can be shown that the maximum values of
l E1l occur at
- β1 zmax = nπ
or = , n = 0, 1, 2, …….. (4.16)
( ) ( )
= n = 0, 1, 2, …….. (4.17)
CASE B
If η2 < η1, Γ < 0. For this case, the locations of lE1l maximum are given by eq. (4.17)
whereas those of lE1l minimum are given by eq. (4.16). All these are illustrated in Fig. (4.2)
Note that
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Fig. (4.2) Standing waves due to reflection at an interface between two lossless media.
1. lH1l minimum occurs whenever there is lE1l maximum and vice versa.
2. The transmitted wave in medium 2 is a purely traveling wave and consequently there
are no maxima and minima in this region.
The ratio of lE1lmax to lE1lmin (or lH1lmax to lH1lmin) is called the standing – wave ratio s; that
is,
| | | | | |
| |
= | |
= | |
(4.18)
or
| | (4.19)
We now consider a more general situation than that in normal incidence. To simplify
the analysis, we will assume that we are dealing with lossless media. In this section we shall
consider the case of oblique incidence. So, we consider two cases.
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A plane incidence is defined as the plane containing the vector indicating the
direction of propagation of the incident wave and normal to the interface.
( )
E (r, t) = Eo cos (k . r – ωt) = Re [ ] (4.20)
Where r = xax + yay + zaz is the radius or position vector and k = kxax + kyay + kzaz is the
wave number vector or the propagation vector. k is always in the direction of wave
propagation. The magnitude of k is related to ω according to the dispersion relation
(4.21)
Thus for lossless media, k is essentially the same as β. With the general form of E as in
equ.(4.20), Maxwell’s equations reduce to
k x E = ωμH (4.22a)
k x E = - ωεE (4.22b)
k.H = 0 (4.22c)
k.E = 0 (4.22d)
showing that (i) E, H, and k are mutually orthogonal and
(ii) E and H lie on the plane
K . r = kxx + kyy + kzz = constant
from equ.(4.22a) the H field corresponding to the E field in equ. (4.20)
(4.23)
This expression E and H in the general form, we can now consider the oblique
incidence of a uniform plane wave at a plane boundary as shown in fig. (4.3). The plane
defined by the propagation vector k and a unit normal vector an to the boundary is
called the plane of incidence. The angle θi between k and an is the angle of incidence.
Again both the incident and the reflected waves are in medium 1 and transmitted
(or refracted wave) is in medium 2. Let
( ) (4.24a)
( ) (4.24b)
( ) (4.24c)
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Where θr is the angle of reflection and θt is the angle of transmission. But, for lossless
media,
√ (4.28a)
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√ (4.28b)
From equ. (4.26) and (4.28a) it is clear that
(4.29)
So, that the angle of reflection equals the angle of incidence as in optics. Also
from equ. (4.27) and (4.28)
√ (4.30)
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( )
(4.33b)
Where √ .
Requiring that and that the tangential components of E and H be continuous
at the boundary z = 0, we obtain
( ) (4.35a)
( ) (4.35b)
|| (4.36a)
or || (4.36b)
|| (4.37a)
or || (4.37b)
Equations (4.36) and (4.37) are called Fresnel’s Equations. Note that the equations
reduce to equ. (4.11) and (4.12) when as expected. Since and are
related according to Snell’s law of equ.(4.30), (4.36) and (4.37) can be written in terms of
by substituting
√ √ ( ) (4.38)
|| || ( ) (4.39)
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angle also known as the Polarizing angle because an arbitrarily polarized incident wave
will be reflected with only the component of E perpendicular to the plane of incidence.
The Brewster angle is obtained by setting ||
when || in equ.(4.36)
||
or ( ) ( ||
)
|| ( )
(4.40)
It is of practical value to consider the case when the dielectric media are not only
lossless, but nonmagnetic as well – i.e., = = . For this the equ.(4.40) becomes
||
→ || √
or || √ (4.41)
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( )
( ) (4.42b)
( )
(4.43a)
( )
( ) (4.43b)
( ) (4.46b)
(4.47a)
or (4.47b)
(4.48a)
or (4.48b)
which are the Fresnel’s equations for perpendicular polarization. From equ.(4.47)
and (4.48), it is easy to show that
(4.49)
When = = 0 equ.(4.47) and equ.(4.48) becomes equ. (4.11) and (4.12).
For no reflection = 0 (or Er = 0). This is the same as the case of total transmission
( = 0). By replacing with the corresponding Brewster angle
or ( ) ( )
incorporating equ.(4.31)
( )
(4.50)
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or √ (4.51)
Total internal reflection refers to the complete reflection of a ray of light within an
optically-denser medium from the surrounding surfaces of optically less dense media
back into the denser medium.
Light ray travel from an optically denser medium to a less dense medium.
The angle of incidence must be greater than a certain angle, called the critical
angle.
For a ray of light passing from an optically denser to a less dense medium, critical
angle, c, is the angle of incidence at which the angle of refraction is 90o.
When the angle of incidence is less than the critical angle, the ray passes out into
the less dense medium.
When the angle of incidence is greater than the critical angle, the ray is reflected
back into the denser medium.
The equation relating critical angle, c and the refractive index, n is:
Sin c=1/n (4.52)
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( ) ( ) (4.55)
From equ.(4.53a)
( ) ( ) ( ) (4.56)
( )
∫ ( ) ∫[ ] ∫ (4.57)
∮( ) ∫[ ] ∫ (4.58)
Equation (4.58) is referred to as Poynting theorem. The quantity E x H on the left hand
side of equ.(4.58) is known as the Poynting vector P in watts per square meter (W/m2).
P=ExH (4.59)
9. PRACTICE QUIZ
1. Electromagnetic waves travel in conductors than in dielectrics?
a) True
b) False
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a) ε
b) μ
c) σ
d) All
4. If the medium 1 is a perfect dielectric, then σ1 = _________
a) -1
b) 1
c) 0
d) ∞
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10. ASSIGNMENTS
S.No Question BL CO
1 Write short notes on: (i) Surface impedance. (ii) Brewster angle 2 4
Explain reflection of uniform plane wave by a perfect conductor in
2 2 4
the case of oblique incidence for parallel polarization
3 State and prove pointing theorem? 2 4
Discuss the determination of the reflected and wave fields of a
4 uniform plane wave incident normally onto a plane boundary 1 4
between two material media.
or ||
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||
or ||
10 Define Brewster angle? 1 4
Ans. ||
From the above equation that ||=0 because the numerator is the
difference of two terms. Under this conditions, there is no reflection
(Ero = 0) and the incident angle at which this takes place is called
the Brewster angle.
S.No Question BL CO
1 Explain the reflection of uniform plane wave with normal incidence 2 3
at plane dielectric boundary?
2 Explain the reflection of uniform plane wave with oblique incidence 2 3
at plane dielectric boundary?
3 Determine the resultant electric and magnetic fields of plane wave 2 3
when it is incident on a perfect conductor normally?
4 Explain the wave motion in perfect dielectric medium? 1 2
5 Discuss about the concept of Poynting theorem 2 3
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