1. Reflection and Transmission of Plane Waves
ECE 3317
Prof. Ji Chen
Spring 2014
Notes 18
Reflection and Transmission of Plane Waves

2. General Plane Wave
Consider a plane wave propagating at an arbitrary direction in space. x y z z
Denote so

3. General Plane Wave (cont.)
Hence x y z z
Note: or
(wavenumber equation)

4. General Plane Wave (cont.)x y z z
We define the wavevector:
(This assumes that the wavevector is real.)
The k vector tells us which direction the wave is traveling in.

5. TM and TE Plane Waves z z S S H TMz TEz H E E y y x x
The electric and magnetic fields are both perpendicular to the direction of propagation.
There are two fundamental cases:
Transverse Magnetic (TMz ) Hz = 0
Transverse Electric (TEz) Ez = 0 x
TMz y z E H S z S
TEz H E y x
Note: The word “transverse” means “perpendicular to.”

6. Reflection and Transmission
As we will show, each type of plane wave (TEz and TMz) reflects differently from a material. #1 x z qi qr qt #2
Incident
Reflected
Transmitted

7. Boundary Conditions
Here we review the boundary conditions at an interface (from ECE 2317).
++++
No sources on interface:
Note: The unit normal points towards region 1.
The tangential electric and magnetic fields are continuous. The normal components of the electric and magnetic flux densities are continuous.

8. Reflection at Interface
Assume that the Poynting vector of the incident plane wave lies in the xz plane ( = 0). This is called the plane of incidence.
First we consider the (x, z) variation of the fields.
(We will worry about the polarization later.)
Note: The sign for the exponent term in the reflected wave is chosen to match the direction of the reflected wave.

9. Reflection at Interface (cont.)
Phase matching condition:
This follows from the fact that the fields must match at the interface (z = 0).

10. Law of Reflection
Similarly,
Law of reflection

11. Snell’s Law
We define the index of refraction:
Snell's law
Note: The wave is bent towards the normal when entering a more "dense" region.

12. Snell’s Law (cont.)
The bending of light (or EM waves in general) is called refraction.
Reflected
Acrylic block
Normal
Incident
Transmitted

13. Example Given: Find the transmitted angle. Air Water
Note that in going from a less dense to a more dense medium, the wavevector is bent towards the normal.
Note: If the wave is incident from the water region at an incident angle of 32.1o, the wave will exit into the air region at an angle of 45o.
Note: At microwave frequencies and below, the relative permittivity of pure water is about 81. At optical frequencies it is about

14. Critical Angle
The wave is incident from a more dense region onto a less dense region. #1 x z qi qr qt #2
Incident
Reflected
Transmitted
qi < c #1 x z
qi = c qr qt #2
Incident
Reflected
Transmitted qi
qt = 90o
At the critical angle:
qt = 90o

15. Example x z Find the critical angle. Water Reflected Incident qc qr #1#2
Incident
Reflected
Transmitted qc
Air

16. Critical Angle (cont.) x z At the critical angle: #1 qi = c qr qt #2
Incident
Reflected
Transmitted qi
qt = 90o
There is no vertical variation of the field in the less-dense (transmitted) region.

17. Critical Angle (cont.) x z Beyond the critical angle: #1 qi > c qr#2
Incident
Reflected qi
There is an exponential decay of the field in the vertical direction in the less-dense region.
(complex)

18. Critical Angle (cont.) x z Beyond the critical angle: qi > c#1 x z
qi > c qr #2
Incident
Reflected qi
The power flows completely horizontally.
(No power crosses the boundary and enters into the less dense region.)
This must be true from conservation of energy, since the field decays exponentially in the lossless region 2.

19. Critical Angle (cont.) Example: "fish-eye" effect Air Water
The critical angle explains the “fish eye” effect
that you can observe in a swimming pool.
A fish can see everything above the water by only looking no further than 49o from the vertical.

20. Exotic Materials
Artificial “metamaterials” that have been designed that have exotic permittivity and/or permeability performance.
Negative index metamaterial array configuration, which was constructed of copper split-ring resonators and wires mounted on interlocking sheets of fiberglass circuit board. The total array consists of 3 by 20×20 unit cells with overall dimensions of 10×100×100 mm.
(over a certain bandwidth of operation)

21. Exotic Materials
The Duke cloaking device masks an object at one microwave frequency.
Image courtesy Dr. David R. Smith.
Cloaking of objects is one area of research in metamaterials.

22. TEz Reflection Ei Hi qi qr #1 x qt #2 z
Note that the electric field vector is in the y direction.
(The wave is polarized perpendicular to the plane of incidence.)

23. TEz Reflection (cont.)
Note: kzr is positive since we have already explicitly accounted for the sign in the reflected wave.

24. TEz Reflection (cont.) Boundary condition at z = 0:
Recall that the tangential component of the electric field must be continuous at an interface.
Boundary condition at z = 0:

25. TEz Reflection (cont.)
We now look at the magnetic fields.

26. TEz Reflection (cont.) Hence we have:
Recall that the tangential component of the magnetic field must be continuous at an interface (no surface currents).
Hence we have:

27. TEz Reflection (cont.) Enforcing both boundary conditions, we have:
The solution is:

28. TEz Reflection (cont.)
Transmission Line Analogy
Incident

29. TMz Reflection Ei Hi qi qr #1 x qt #2 z
Note that the electric field vector is in the xz plane.
(The wave is polarized parallel to the plane of incidence.)
Word of caution: The notation used for the reflection coefficient in the TMz case is different from what is in the Shen & Kong book. (We use reflection coefficient to represent the reflection of the electric field, not the magnetic field.)

30. TMz Reflection (cont.)

31. TMz Reflection (cont.) We now look at the electric fields.
Note that TM is the reflection coefficient for the tangential electric field.

32. TMz Reflection (cont.) Boundary conditions:
Enforcing both boundary conditions, we have
The solution is:

33. TMz Reflection (cont.)
Transmission Line Analogy
Incident

34. Summary of Transmission Line Modeling Equations
TMz Reflection (cont.)
Summary of Transmission Line Modeling Equations
Incident

35. Power Reflection

36. Power Reflection Beyond Critical Angle#1 x z
qi > c qr #2
Incident
Reflected qi
All of the incident power is reflected.

37. Example qi qr #1 x qt #2 z Find:
Given:
Find:
% power reflected and transmitted for a TEz wave
% power reflected and transmitted for a TMz wave
Snell’s law:

38. Example (cont.)
First look at the TMz case:

39. Example (cont.)
Next, look at the TEz part:

40. Example qi qr #1 x qt #2 z Find:
Given: #1 x z qi qr qt #2
Sea water
Find:
% power reflected and transmitted for a TEz wave
% power reflected and transmitted for a TMz wave

41. Example (cont.) qi qr #1 x qt #2 z Given: Sea water
We avoid using Snell's law since it will give us a complex angle in region 2!

42. Example (cont.) qi qr #1 x qt #2 z Given: Sea water
Recommendation: Work with the wavenumber equation directly.
complex

43. Example (cont.)
First look at the TMz case:

44. Example (cont.)
Next, look at the TEz part:

45. Brewster Angle
Consider TMz polarization
Assume lossless regions
Set

46. Brewster Angle (cont.)
Hence we have

47. Brewster Angle (cont.)
Assume m1 = m2:

48. Brewster Angle (cont.)
q i
Geometrical angle picture:
Hence

49. Brewster Angle (cont.)
This special angle is called the Brewster angle b.
For non-magnetic media, only the TMz polarization has a Brewster angle.
A Brewster angle exists for any material contrast ratio (it doesn’t matter which side is denser).

50. Brewster Angle (cont.)
Example
Air
Water

51. Brewster Angle (cont.) Polaroid Sunglasses TMz+TEz TEz Eye Sunlight
Polarizing filter
(blocks TEz)
TMz+TEz
Sunlight
Puddle of water
TEz
Eye
The reflections from the puddle of water (the “glare”) are reduced.